Aeration

Oxygen is the one thing koi cannot wait for. Warm water holds less of it exactly when the fish and the filter bacteria want more, which is why summer nights are when ponds are lost. Linear air pumps from Hakko and Alita, plus membrane diffusers, air stones, manifolds and airline to distribute it.

Size on roughly 1 CFM per 1,000 gallons as a floor, and 1.5 to 2 if your pond is deep, heavily stocked or sits in a hot climate. Diaphragms are a wear part on every linear pump, so keep a rebuild kit on the shelf.

Rubber air diffuser disc, two spherical air stones, a brass valve manifold and coiled airline on a stone ledge beside a koi pond, used to oxygenate water and filter media.

Size it right the first time

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

Aeration: size by fish load, not just gallons

Plan on roughly 1 CFM of air per 1,000 gallons as a floor, and 1.5 to 2 CFM if your pond is deep, heavily stocked, or sits in a hot climate. Depth matters as much as volume, because the pump has to overcome the water pressure above the diffuser. The chart below is Hakko's own recommendation and reads across to the equivalent Alita models.

Swipe the table to see heavy stocking →

Pond volumeLight fish loadMedium fish loadHeavy fish load
1,000 to 4,000 gal40 L40 L60 L
4,000 to 8,000 gal60 L80 L100 L
8,000 to 12,000 gal80 L100 L120 L
12,000 to 20,000 gal100 L120 LTwo 80 L units

Under 1,000 gallons, the Alita AL-6A or AL-15A is the right scale. Diaphragms are a wear part on every linear air pump: keep a rebuild kit on the shelf rather than losing a weekend to a dead pump.

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.

Common questions

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.

I have a waterfall. Do I still need an air pump?＋

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.

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