Collection: Pond Supplies

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 higher up this page does the arithmetic and turns the answer into pump, UV and air recommendations.

If it is pea-soup green and you cannot see your hand a few inches down, that is suspended single-celled algae and yes, a correctly sized UV is the reliable fix. Most ponds clear within three to five days once it is running. Plumb it after the filter so the water reaching it is already clear, because UV-C cannot penetrate dirty water.

What a UV will not do is stop the algae coming back if the underlying cause is untouched. Green water is fed by nutrients from fish waste, overfeeding and sunlight, so a UV is a control, not a cure.

No, and this is the most common misunderstanding we hear. String algae is filamentous and it clings to surfaces, so it never passes through the UV chamber. Running a bigger UV will do nothing for it.

String algae is a nutrient problem. Pull it out by hand where you can, cut back on feeding, keep up water changes, and use an algaecide or a barley product to buy yourself time while the real fix takes hold. Ironically, ponds with excellent UV often get more string algae, because the free-floating algae is no longer competing for the same nutrients.

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.

Bigger than you think. Friction loss climbs steeply as pipe gets narrower, and every foot of flow you lose to undersized plumbing is flow you paid for at the pump. As a working guide: 1-1/2 inch for pressurized returns on smaller systems, 2 inch for most koi pond returns, and 3 to 4 inch for gravity-fed bottom drain lines, which move water on almost no pressure and need the diameter.

Use sweeping 45 degree bends instead of sharp 90s wherever you can, and put a union either side of every serviceable component.

Submersibles are cheaper up front, simple to install, and perfectly good on ponds under about 1,500 gallons or on a separate waterfall circuit. The downsides are that electricity sits in the water, and servicing means reaching in.

External pumps cost more but are usually cheaper to run over a season, last longer, and can be serviced without getting wet. On a koi pond of any real size, an external pump is the standard choice. One catch: most are not self-priming and must sit below the water line so the intake stays flooded. If yours has to sit higher, you need a self-priming model.

Yes. A waterfall adds oxygen, but it does it only at the surface and only while the main pump is running. An air pump puts oxygen in at the bottom, drives circulation through the whole water column, and keeps working when the main pump fails or you shut it down to service the filter.

It is the cheapest insurance in the hobby. Warm summer nights are when oxygen bottoms out and when ponds are most often lost.

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.

That is almost always low KH, or carbonate hardness. KH is the buffer that holds pH steady, and koi ponds consume it continuously through the nitrification process. When it runs down far enough, pH stops being anchored and swings hard between day and night, which is far more stressful for fish than a stable pH at the wrong number.

Test KH, not just pH. If it is low, buffer it. Chasing pH with pH-adjusting products while KH is on the floor makes the swings worse, not better.

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.