Filters & Media

Mechanical filtration catches what you can see. Biological filtration converts what you cannot. Pressurised bead filters, Bakki showers, skimmers, Japanese matting and honeycomb media, plus the O-rings and gaskets that keep it all sealed.

Size to your fish load, not just your gallons: a heavily stocked 4,000 gallon pond is happier on a filter rated for 6,500. Nobody has ever regretted an oversized filter, and you cannot fix an undersized one with chemicals.

Japanese filter matting, a honeycomb biological media cylinder and white filter beads on a stone ledge beside a koi pond, the media that convert ammonia in a koi filter.

Size it right the first time

Manufacturers' own numbers, the same charts behind our pond sizing calculator.

How many koi will it actually hold?

Stocking is where beginners get caught out, because tosai are small and adults are not. Plan for the fish you are buying at full size. These are working numbers for a properly filtered pond with good aeration; a lightly filtered pond should stay at the conservative end.

Swipe the table to see what it means →

Koi sizeWater per fishWhat that means
Tosai, under 10 in~50 galEasy to over-buy at this stage. They will not stay this size.
Nisai to sansai, 12 to 20 in~150 galThe size most hobby ponds are really stocked at.
Adult, 24 in and up250 to 350 galA 3,000 gallon pond comfortably holds around ten adult koi.

Overstocking does not usually kill fish outright. It stunts growth, keeps ammonia and nitrite twitching, and turns every minor parasite into an outbreak. If you are near the limit, add filtration and aeration before you add fish.

Worth knowing before you buy

Ground rules and answers from the ponds we run every day in San Jose.

Know your real gallons

Every number in this hobby is dosed or sized by volume. Measure once, write it on the filter housing in marker, and stop guessing. Most people overestimate by a third.

Buy one size up on filtration

Nobody has ever regretted an oversized filter. Undersizing is the most common and most expensive mistake in the hobby, and you cannot fix it with chemicals.

Dechlorinate every refill

Chlorine and chloramine kill the bacteria colony you spent two months growing, and damage gills on the way through. Treat the water going in, every single time.

Common questions

How do I work out how many gallons my pond holds?＋

Length times width times average depth, all in feet, times 7.5 for a rectangular pond. For a round or oval pond use 5.9, and for an irregular shape use about 6.7. Average depth matters: measure it in a few places and take the mean rather than using the deepest point, or you will overestimate badly.

The calculator on our Pond Supplies guide does the arithmetic and turns the answer into pump, UV and air recommendations.

Can I turn the pump off at night or in winter?＋

Not at night, no. The bacteria in your biofilter need constant oxygenated flow, and they start dying within hours of losing it. Turning the pump off overnight is one of the fastest ways to wreck a mature filter, and night is also when oxygen is at its lowest.

Winter is a different conversation and depends on where you are. In hard-freeze climates many keepers shut the main pump and filter down, and run an air pump instead to hold a gas exchange hole open in the ice. In mild climates, keep everything running year round.

How much water should I change, and how often?＋

Around 10 percent a week is a good habit for a stocked koi pond, either in one go or trickled continuously. Water changes are how nitrate and dissolved organics actually leave the system, and they replace the carbonate hardness your pond consumes.

Always dechlorinate. If you are adding a large volume at once, match the temperature reasonably closely and treat the incoming water rather than dosing the pond afterwards.

I am not sure what I need. Can you just tell me?＋

Gladly, and this is genuinely the fastest route. Send us your pond volume, your depth, roughly how many koi you keep and how big they are, your plumbing size, and the height from the pond surface up to your filter return. That is everything we need to size a system properly.

We have been building and maintaining koi ponds from San Jose since 1998, and we run this gear on our own systems. We would rather talk you into the right pump than sell you the biggest one.

Have us size your systemFull pond guide & calculator