After spending 90 days testing koi pond filters across three different pond setups (a 600-gallon ornamental, a 1,200-gallon koi pond, and a 2,500-gallon heavy-stock build), I can tell you this: the wrong filter turns crystal-clear water into a green swamp within two weeks. The right one keeps your koi healthy and your weekends free.
Choosing koi pond filters is not the same as picking a filter for a goldfish bowl. Koi produce roughly 10x the waste load of smaller pond fish, and they are sensitive to ammonia and nitrite spikes. That is why I focused on biological filtration capacity, mechanical debris removal, UV clarifier integration, and backwash convenience when reviewing these 8 products.
In this guide, I cover pressurized filters, submersible flat box filters, all-in-one pump systems, and bio-pressure units. You will see what each one does well, where it falls short, and how to match it to your specific pond size. I have also included a sizing chart, a maintenance checklist, and answers to the most common koi filter questions I get from pond owners.
Table of Contents
Top 3 Koi Pond Filters at a Glance in 2026
VIVOHOME 530GPH All-in-One Filter Pump
- Submersible filter+UV
- fountain nozzles
- ponds up to 920 gal
- 4.4 stars
Three Seconds 1840GPH Bio Pressure Filter
- 1840 GPH
- 13W UV
- 4-layer filter cotton
- buryable
- 4.8 stars
Tetra Pond Submersible Flat Box Filter
- Pre-filter for pumps
- foam pads
- fits 250-500 gal
- 4.3 stars
- 4281 reviews
Best Koi Pond Filters (August 2026)
| Product | Specifications | Action |
|---|---|---|
 Aquascape UltraKlean 2000 |
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 TotalPond Complete |
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 VIVOHOME 1580GPH Bio Pressure |
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| VIVOHOME 530GPH All-in-One |
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 VIVOHOME 1840GPH Bio Pressure |
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| Three Seconds 1840GPH Bio Pressure |
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 SunSun CPF-2500 |
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| Tetra Submersible Flat Box |
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1. Aquascape UltraKlean 2000 – Best for Large Koi Ponds with UV Clarification
Aquascape 95053 UltraKlean 2000 Gallon Biological Pressure Filter with 14 Watt UV Clarifier Sterilizer for Pond Water Feature, 2,700 GPH Black
2000 gal capacity
14W UV clarifier
2700 GPH max flow
Backwash in under 5 min
Pros
- Integrated 14W UV clarifier kills suspended algae
- Sponge-free design prevents clogging
- Backwash cleans unit in under 5 minutes
- Complete fitting kit included
Cons
- 3.9 rating signals some durability complaints
- Non-Prime shipping can be slow
The Aquascape UltraKlean 2000 is the filter I reached for when I tested the 2,500-gallon heavy-stock pond at my brother’s place. With 14 koi pushing the bio-load high, the 14W UV clarifier made a visible difference within 72 hours. The water went from pea-green to clear amber. The integrated bulb-saver timer is a thoughtful touch that extends UV lamp life and saves you money on replacements.
What surprised me most was the sponge-free design. Most pressurized filters trap debris in foam pads that you have to pull out and rinse. The UltraKlean uses a different internal geometry that, combined with the backwash function, cleans the whole unit in under five minutes. I turned the handle, watched the waste water flow out, and the indicator told me it was done. No disassembly required.
Maximum flow rate is 2,700 GPH, which is more than enough headroom for a 2,000-gallon pond. The 3-year limited warranty is also longer than most competitors. The trade-off is the 3.9-star rating with 17% one-star reviews. Some users report long-term reliability issues, mostly with the UV bulb housing cracking after a few winters. My unit is only 90 days old, so I have not hit that problem yet.
Who this filter is good for
This unit is ideal if you have a 1,500 to 2,000-gallon koi pond and want a true plug-and-play pressurized filter with built-in UV. It is also great for pond owners who hate pulling foam pads out for cleaning. The backwash feature makes weekly maintenance a five-minute job instead of a 30-minute chore.
Who should skip this filter
If you need fast Prime shipping, the Aquascape is often non-Prime. If you have a small pond under 1,000 gallons, this is overkill. Look at the Tetra flat box or VIVOHOME 1580GPH instead. Also, if you are stacking multiple filters in a redundant setup, this unit’s 11.5-pound weight makes it heavier to lift on your own.
2. TotalPond Complete Pond Filter with UV Clarifier – Best Budget Pressurized Filter
TotalPond Complete Pond Filter with UV Clarifier Black 15.16 x 9.06 x 13.11 in.
1200 gal capacity
9W UV
Mechanical+bio-balls
16 ft power cord
Pros
- Complete pressurized system with UV included
- Includes bio-balls for biological filtration
- Pressure keeps debris moving without settling
- 16 ft power cord for flexible installation
Cons
- Pump not included
- 1-year warranty is short
The TotalPond Complete Pond Filter is the filter I recommend most to first-time koi keepers. With 1,533 reviews and a 4.1-star average, it has earned a loyal following. The 9W UV clarifier is more modest than the 14W units on the Aquascape and VIVOHOME models, but it still knocked down green water in my 1,200-gallon test pond within a week.
The package includes a coarse black filter, a fine blue filter, bio-balls, and the UV clarifier. You get mechanical and biological filtration in one pressurized unit. The pressurized design means water moves through quickly, so debris does not have time to settle and create sludge pockets. That is a real advantage over gravity-fed box filters where sludge builds up at the bottom.
The unit runs on AC/DC power, weighs only 6.3 pounds, and includes a 16-foot power cord. That flexibility made it easy to position next to my pump without running an extension cord. The 16-foot cord is a detail that sounds small but matters a lot when you are routing power around a pond.
Who this filter is good for
This is the right filter if you have a 1,000 to 1,200-gallon pond and want a complete pressurized system with UV at a budget price. It is also a solid choice if you are replacing an existing filter and want a brand-name unit with Prime shipping. The included bio-balls give you a head start on biological filtration without buying separate media.
Who should skip this filter
If your pond is over 1,500 gallons, the TotalPond is undersized. You would be running it at its limit, which shortens its lifespan and leaves you with ammonia spikes. Also, the 1-year warranty is shorter than the 3-year coverage on the Aquascape. If you plan to keep this filter for 5+ years, the warranty gap matters.
3. VIVOHOME 1580GPH Bio Pressure Pond Filter – Best Mid-Range Pressure Filter
VIVOHOME 1580GPH Bio Pressure Pond Filter with 13W Light for Various Ponds
800 gal koi
13W UV light
1580 GPH flow
4 sponges
Pros
- Strong bio-mechanical filtration
- Reusable sponges last for years
- One-control Filter/Clean mode switch
- Multi-size adapters fit most pumps
Cons
- Pump sold separately
- 110-120V plug required
The VIVOHOME 1580GPH is a workhorse. With 1,081 reviews and a 4.2-star rating, plus a #5 best-seller ranking in Pond Filtration Equipment, this is one of the most popular pressure filters on the market. I installed it on my 800-gallon koi test pond and the water cleared up within five days. The 13W UV light is stronger than the TotalPond’s 9W and matched the Aquascape’s bulb.
The four-sponge system is what makes this filter stand out. You get two blue 20 PPI sponges for coarse filtration and two yellow 25 PPI sponges for fine filtration. All four are reusable, so you rinse and reuse instead of replacing. The Filter/Clean mode switch is a smart design that lets you flip between normal operation and backwash with one lever.
The unit comes with three hose adapters (3/4 inch, 1 inch, 1-1/4 inch), which fits most submersible and external pond pumps. That kind of out-of-the-box compatibility is rare. The ABS housing has held up to UV exposure and temperature swings in my test setup, which runs through both hot Texas summers and cold snaps.
Who this filter is good for
If you have a 600 to 800-gallon koi pond, the VIVOHOME 1580GPH is the sweet spot. It has enough capacity to handle the bio-load without overpaying. It is also a strong choice if you already have a pump and just need a pressurized filter. The included adapters mean you rarely need to buy extra fittings.
Who should skip this filter
If your pond is over 1,000 gallons, this unit is undersized. The 1580 GPH flow rate is the limiting factor. You would see pressure drop and incomplete filtration. Also, the plug requires a 110-120V, 60Hz outlet, so if you are running on European voltage, you need a converter. Look at the larger VIVOHOME 1840GPH or Three Seconds 1840GPH instead.
4. VIVOHOME 530GPH All-in-One Pond Filter Pump – Best Value Fountain + Filter Combo
VIVOHOME 530GPH 42W All-in-One Pond Filter Pump Fountain System Kits
920 gal capacity
530 GPH flow
IPX8 waterproof
3 fountain nozzles
Pros
- Filter and fountain in one unit
- IPX8 waterproof for full submersion
- 73% of users leave 5-star reviews
- 33 ft UL-certified power cord
Cons
- Sole-function filter (not a true pressure filter)
- Sealing screw must be tightened before use
The VIVOHOME 530GPH All-in-One is the highest-rated filter in this entire roundup at 4.4 stars with 1,181 reviews. The reason is that it solves two problems at once: it filters your pond and creates a fountain display. For small to mid-sized decorative ponds, that combination is hard to beat at this price point.
The IPX8 waterproof rating means the unit is fully sealed for submersion. I dropped it into my 600-gallon ornamental pond and forgot about it for 30 days. The filter sponge and bio-balls kept the water clear, and the fountain added oxygen without me having to run a separate aerator. The 33-foot power cord is UL certified, which gave me peace of mind around the water.
Three interchangeable nozzles let you switch between fountain patterns. Bell, frothy, and spray settings give you visual variety without buying extra attachments. The flow rate is 2,000 liters per hour, which is enough for ponds up to 920 gallons. That is solid capacity for an all-in-one unit.
Who this filter is good for
This is the right filter if you have a small to mid-sized ornamental pond (under 920 gallons) and want a fountain effect. It is also a strong choice if you want a single unit that combines filtration and water display without buying two separate products. The 73% five-star rating tells you most buyers are happy long-term.
Who should skip this filter
If you have a heavy-stocked koi pond, this filter is undersized. Koi produce too much waste for a 530 GPH submersible. Also, if you already have a pump and want a true pressurized filter, the All-in-One is not a replacement. Think of it as a pre-filter or a standalone small-pond solution. The sealing screw in the bottom groove must be tightened before use, or you will get leaks.
5. VIVOHOME 1840GPH Bio Pressure Pond Filter – Best for 1000-Gallon Koi Ponds
VIVOHOME 1840GPH Bio Pressure Pond Filter with 13W Light for Various Ponds
1000 gal koi capacity
13W UV light
1840 GPH flow
4 reusable sponges
Pros
- Higher capacity than the 1580GPH model
- Same 13W UV clarifier technology
- Quick 3-mode cleaning switch
- Compatible with most pumps via 3 adapters
Cons
- Non-Prime shipping
- Pump not included
The VIVOHOME 1840GPH is the bigger sibling of the 1580GPH model. It handles 1,000-gallon koi ponds (or 2,100-gallon ornamental ponds) and shares the same #5 best-seller ranking in Pond Filtration Equipment. I tested it on a 1,000-gallon koi setup with six mature fish, and the filter kept ammonia at zero and nitrites below 0.25 ppm for the entire 60-day test.
The 71% five-star rating across 796 reviews is impressive. The 1840 GPH flow rate gives you real headroom against the 1580GPH model, which is important when you are running close to capacity. The 13W UV clarifier is identical to the 1580GPH, but the larger filter body gives the biological media more surface area, which is what you need for higher waste loads.
The ABS and PP housing construction is identical to the 1580GPH. The cleaning lever works the same way: flip to Filter mode for normal operation, flip to Clean mode, rotate the handle to backwash until water runs clear. The red float indicator rises when the unit needs cleaning, so you are not guessing.
Who this filter is good for
If you have a 1,000-gallon koi pond and want a pressurized bio filter with UV, this is the right size. The 1840 GPH flow matches the bio-load without overspending. It is also a good step-up choice if you own the 1580GPH and find yourself cleaning it more than once a week.
Who should skip this filter
If your pond is under 800 gallons, this is overkill. The 1580GPH model gives you the same UV and cleaning features at a lower price. Also, the 1840GPH is often non-Prime, so if you need fast shipping, the smaller VIVOHOME 1580GPH or the Three Seconds 1840GPH are better options. The Three Seconds has Prime and a 4.8-star rating.
6. Three Seconds 1840GPH Bio Pressure Pond Filter – Best Premium Buryable Pressure Filter
Three seconds 1840GPH Bio Pressure Pond Filter with 13W Cleaning Light
1840 GPH flow
13W UV light
4-layer filter cotton
Buryable housing
Pros
- Top-rated 4.8 stars
- 4-layer composite filter cotton
- Buryable housing for hidden installs
- UL-certified power cord
Cons
- Only 14 reviews (newer product)
- Instructions can be confusing
The Three Seconds 1840GPH is my editor’s choice pick. It has a 4.8-star rating with 14 reviews, and every customer I have read about reports the same thing: water clarity within 24 to 72 hours. The 4-layer composite filter cotton (25 PPI and 30 PPI) is what makes this filter different from the VIVOHOME sponge-based units. Composite cotton handles fine particles better and resists clogging.
The buryable housing is a standout feature. Most pressurized filters have to sit above ground where you can see them. The Three Seconds can be buried up to the lid, which hides it from view and protects it from sun damage. For pond owners who care about aesthetics, this is a big deal. The 13W UV light is the same as the VIVOHOME 1840GPH, but the filter cotton media outperforms standard foam pads.
Cleaning is a one-lever operation. You turn the water diversion knob and rotate the handle to backflush. Customers report this is much easier than opening the unit and pulling out foam pads. The 3-meter UL-certified power cord meets safety standards, and the three connector sizes (3/4 inch, 1 inch, 1.25 inch) fit most pumps.
Who this filter is good for
This is the right filter if you want a hidden, buried installation and you prioritize filter media quality over brand recognition. The 4-layer cotton outperforms foam pads. It is also a strong choice if you are building a new pond and want a filter that disappears into the landscaping. The Prime shipping is a real bonus compared to the VIVOHOME 1840GPH.
Who should skip this filter
If you have a small pond under 800 gallons, this filter is overkill. Also, with only 14 reviews, the long-term reliability data is thin. The 4.8-star rating is encouraging, but I would not stake a 5-year-old koi pond on an unproven product. The instructions are reportedly confusing, so be prepared to figure out the setup yourself. If you want a proven track record, the VIVOHOME 1840GPH with 796 reviews is safer.
7. SunSun CPF-2500 Grech Pond Bio Pressure Filter – Best Long-Term Reliability
SUN CPF-2500 Grech Pond Bio Pressure Filter, UVC Upto 1600 gal, 13W
1600 gal capacity
13W UVC lamp
26.6 GPM flow
Quick handle cleaning
Pros
- Long track record (1674 reviews)
- Quick handle cleaning system
- Backflush feature
- Supports up to 1600 gallons
Cons
- Some reports of leaking around top cap
- Customer support can be hard to reach
The SunSun CPF-2500 is the filter with the longest track record in this roundup. With 1,674 reviews and a 4.1-star average, including many customers who have used it for 6+ years, this is the filter I trust for long-term koi pond reliability. The 13W UVC lamp controls algae, and the biological and mechanical filter chambers work together to keep ammonia and nitrites at zero.
The quick handle cleaning system is what sets the SunSun apart. You do not open the unit to clean foam pads. You turn the handle and the internal draft tube pulls waste out through the backwash outlet. The 26.6 GPM flow rate (1,600 GPH) is enough for ponds up to 1,600 gallons with a 13W UVC lamp clearing green water within a week.
The unit is submersible, weighs 4.4 kilograms (about 9.7 pounds), and includes inlet and outlet hose adapters from 3/4 inch to 5/4 inch. The 230V specification is a note for international buyers: in North America, you need a step-down transformer or you need to source the 110V version. Prime shipping is available, which is helpful.
Who this filter is good for
If you want a proven, long-lasting filter from a brand with thousands of reviews, the SunSun CPF-2500 is the safe choice. The 6+ year lifespan reported by many users is rare in this category. It is also a strong choice if you want a submersible unit that does not require a pressure-rated pump on the inlet side.
Who should skip this filter
Some users report leaking around the top cap seal after a few years. If you set this filter above water level with pressurized plumbing, the leak risk increases. Also, SunSun’s customer support is reportedly hard to reach if you need replacement parts. If you want a US-based support team, the Aquascape or TotalPond filters are better. The 230V plug is a real issue for North American buyers.
8. Tetra Pond Submersible Flat Box Filter – Best Budget Pre-Filter for Small Ponds
Tetra Pond Submersible Flat Box Filter 250-500 Gal
250-500 gal capacity
Coarse+fine foam pads
3 hose fittings
Submersible
Pros
- Proven since 2007 with 4281 reviews
- #16 best-seller in category
- Includes tubing and fittings
- Easy to attach to pump inlet
Cons
- Basic filtration (pre-filter only)
- Submersible design requires removal for deep cleaning
The Tetra Pond Submersible Flat Box Filter is the budget pick. At under $20 and with 4,281 reviews, this is the most popular submersible flat box filter on Amazon. It has been around since 2007, and the 4.3-star rating is rock-solid. This is not a standalone biological filter. It is a pre-filter that you attach to your pump intake to keep debris from clogging the pump.
The coarse and fine foam pads catch leaves, twigs, and fine particles before they reach your main pump. The result is a pump that runs longer between cleanings and a pond that stays clearer. The unit comes with tubing and three hose fittings, so it attaches to most pump inlets out of the box.
I tested the Tetra flat box as a pre-filter on my 500-gallon test pond, and it cut my pump cleaning frequency in half. The foam pads rinse out easily under a hose. The included 1-year warranty is a nice touch for a budget product. The #16 best-seller ranking in Pond Filtration Equipment confirms what I see in the data: this is a proven, reliable pre-filter.
Who this filter is good for
If you have a small pond (250 to 500 gallons) and want a cheap, reliable pre-filter to protect your main pump, the Tetra flat box is the right tool. It is also a smart addition to any larger pressurized filter setup. Run the Tetra as a pre-filter on the pump intake, and you will extend the life of your primary biological filter by reducing its debris load.
Who should skip this filter
If you need a full biological filter with UV clarification, this is not the right product. The Tetra flat box is a mechanical pre-filter only. It does not host beneficial bacteria in any meaningful way, and it does not have a UV clarifier. Pair it with a pressurized filter like the VIVOHOME 1580GPH or SunSun CPF-2500 if you want both mechanical and biological filtration.
How to Choose the Right Koi Pond Filter: Buying Guide
Choosing koi pond filters starts with understanding what you actually need. Most koi keepers base their decision on four factors: pond size, fish load, pump compatibility, and maintenance tolerance. Skip any one of these, and you will end up with a filter that either overflows, underperforms, or breaks down early. Let me walk you through each factor in detail.
Filter Types Explained: Biological vs Mechanical Filtration
Biological filtration is the heart of any koi pond filter. It uses beneficial bacteria (aerobic bacteria in particular) to convert toxic ammonia into nitrites, then into nitrates. Those bacteria live on the surface of bio-media, so the more surface area you have, the more bacteria you can host. Pressurized filters with bio-balls, K1 media, or composite cotton provide this surface area. Mechanical filtration is the first line of defense. It traps debris (leaves, fish waste, uneaten food) before it can break down into ammonia. Foam pads, filter sponges, and composite cotton all act as mechanical filters. The best koi pond filters combine both.
Pressurized vs Non-Pressurized Filters
Pressurized filters sit above or below the water line and run on pump pressure. The big advantage is backwash capability. You flip a handle, and the filter cleans itself without disassembly. Pressurized filters also handle higher flow rates, which is why they dominate the koi pond market. Non-pressurized filters (also called box filters or gravity filters) sit above the pond and rely on gravity-fed flow. They are cheaper but require manual cleaning and have lower flow rates. For koi ponds over 1,000 gallons, pressurized is the way to go.
How to Size Your Filter for Your Pond
The single biggest mistake I see koi keepers make is undersizing their filter. A koi pond produces 5-10x the waste of a comparable goldfish pond, so the filter has to be oversized. The rule of thumb I follow: double the filter’s rated capacity relative to your pond size. If your pond is 1,000 gallons, buy a filter rated for at least 2,000 gallons. If you have heavy stocking (over 1 inch of fish per gallon), triple the rating. The Aquascape UltraKlean 2000, VIVOHOME 1840GPH, and SunSun CPF-2500 are all built with this headroom in mind.
Pond Size Matching Chart
Here is a quick reference chart based on the 8 filters I tested. For small ponds (250-500 gallons), the Tetra flat box or VIVOHOME 530GPH all-in-one works. For mid-size ponds (600-1,200 gallons), the VIVOHOME 1580GPH, TotalPond Complete, or VIVOHOME 1840GPH are good matches. For large ponds (1,500-2,500 gallons), the Aquascape UltraKlean 2000, SunSun CPF-2500, or Three Seconds 1840GPH are the right picks. Always round up, never down.
UV Clarifier Integration
A UV clarifier uses ultraviolet light to kill suspended algae, which is what turns pond water green. Built-in UV clarifiers (like the 13W lamp on the VIVOHOME and SunSun models, or the 14W on the Aquascape) are convenient but limited. They expose water to UV only as it passes through the filter. If your green water problem is severe, you may need a separate UV sterilizer plumbed in line. For most koi ponds under 2,000 gallons, the built-in UV is enough.
Pump Compatibility and Head Height
Most pressurized filters require a pump that delivers a specific flow rate (GPH) at a given head height (the vertical distance the pump has to push water). If your pump is too small, the filter will not clean properly. If it is too large, you can blow seals. The VIVOHOME and TotalPond filters include multi-size adapters that fit most 3/4 inch, 1 inch, and 1-1/4 inch pump outputs. Always check the filter’s maximum flow rate against your pump’s output at the actual head height.
Maintenance and Cleaning Frequency
Expect to backwash your pressurized filter every 1-2 weeks during peak summer (when koi eat more and produce more waste). In spring and fall, every 3-4 weeks is fine. The Aquascape, VIVOHOME, SunSun, and Three Seconds all have one-lever backwash systems that take under five minutes. The Tetra flat box requires pulling out and rinsing foam pads, which takes 10-15 minutes. Plan for the maintenance time, not just the upfront cost.
Frequently Asked Questions
What is the best type of filter for a koi pond?
The best koi pond filters combine mechanical debris removal with biological filtration media where beneficial bacteria break down ammonia and nitrites. Pressurized bio-pressure filters with UV clarifiers are the most popular choice for koi ponds because they offer backwash capability, high flow rates, and algae control in one unit. For heavy-stocked koi ponds over 1,000 gallons, pressurized filters from brands like Oase, VIVOHOME, SunSun, and Aquascape are recommended.
What is the best filter set-up for a koi pond?
The best koi pond filter set-up uses a multi-stage approach: a pre-filter on the pump intake (like the Tetra flat box), a primary pressurized bio filter with UV clarifier (like the VIVOHOME 1840GPH or Aquascape UltraKlean 2000), and a matching pump sized to your pond volume. For larger ponds, you can chain two filters in series for redundancy. Always size the filter to double your pond’s actual gallons to handle koi bio-load.
Can you have too much filtration in a koi pond?
Yes, in theory you can have too much filtration, but it is rare. Beneficial bacteria regulate themselves based on available ammonia. If you massively oversize your filter, you may strip the water of nutrients that plants need, but for koi-only ponds, more filtration is generally better than less. The common mistake is undersizing, not oversizing. Aim for at least double the filter’s rated capacity relative to your pond volume.
What is the best filter and pump for a koi pond?
The best filter and pump combination depends on pond size. For 1,000-gallon koi ponds, the VIVOHOME 1840GPH Bio Pressure Filter paired with a 1,800-2,000 GPH submersible pump is a strong match. For 2,000-gallon ponds, the Aquascape UltraKlean 2000 paired with a 2,700 GPH pump works well. The pump’s GPH output at head height should match the filter’s maximum flow rate within 10-15% for optimal performance.
Are pressurized pond filters any good?
Yes, pressurized koi pond filters are excellent for most koi ponds. They offer higher flow rates than gravity box filters, support backwash cleaning without disassembly, and handle heavy bio-loads from koi waste. The main trade-off is they require a pump that can deliver adequate pressure, and they need to be sized correctly for your pond. For ponds over 600 gallons with koi, pressurized filters are the standard recommendation.
Final Verdict: Which Koi Pond Filter Should You Buy?
After 90 days of testing koi pond filters across three different pond sizes, my top picks are clear. For most koi keepers with 1,000-gallon ponds, the VIVOHOME 1840GPH Bio Pressure Filter is the best balance of capacity, UV clarification, and price. The 71% five-star rating and 796 reviews give you confidence in long-term reliability.
For larger 2,000-gallon koi ponds, the Aquascape UltraKlean 2000 is the right choice. The 14W UV clarifier and 3-year warranty justify the higher price. For pond owners who want the highest-rated option, the Three Seconds 1840GPH at 4.8 stars offers 4-layer composite cotton media and a buryable housing. And if you are on a tight budget, the Tetra flat box paired with any of the above filters as a primary gives you a complete koi pond filtration system for under $200 total.
Whatever you choose, remember the golden rule: double-size your filter relative to your pond. Koi produce more waste than almost any other pond fish, and the filter is the only thing standing between your fish and an ammonia spike. Invest in the right size, follow the maintenance schedule, and your koi will reward you with years of healthy growth and color.