Test Kits

Ammonia and nitrite are invisible, odorless and lethal, and by the time a fish is flashing or sitting on the bottom you have lost a week you did not need to lose. A test kit is the only way to see the problem while it is still cheap to fix. Master kits from API and PondCare cover pH, ammonia, nitrite and nitrate in one box, and single parameter kits let you replace whichever reagent you run through fastest.

Two habits worth building. Test KH alongside pH, because pH on its own tells you where you are but not whether you are about to fall: koi ponds consume carbonate hardness continuously, and a pond that has run out will swing hard overnight. And test before you medicate, every time. Most sick looking koi are stressed by their water rather than infected, and a treatment dosed on top of bad water usually makes things worse. Liquid reagent kits take a few minutes longer than strips and are considerably more accurate, so when a reading is going to decide whether you dose the whole pond, use the liquid.

Color-coded pond water test vials beside a koi pond used for measuring pH, ammonia, nitrite, and nitrate levels to maintain healthy koi fish.

Start here: two kits test everything that matters

You do not need a shelf full of chemistry. These two kits between them read every number a koi pond runs on. The single-parameter kits below are refills for whichever test you use up first.

Kit 1 of 2: the nitrogen cycle
API Freshwater Master Test Kit in a clear carrying case with blue lid, tests pH, ammonia, nitrite and nitrate
pH High range pH Ammonia (NH3) Nitrite (NO2) Nitrate (NO3)

Ammonia, nitrite and nitrate are the three numbers that tell you whether your filter is keeping up with your fish, and pH is the number they all depend on. This one box runs all five tests, roughly 800 of them, and it is the kit we reach for first on our own ponds in San Jose.

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Kit 2 of 2: hardness and stability
API GH and KH general and carbonate hardness test kit shown in its gray retail box
General hardness (GH) Carbonate hardness (KH)

KH is the buffer that holds pH steady, and a koi pond consumes it continuously through nitrification. When KH runs out, pH swings hard between morning and night, which stresses fish far more than a stable pH at the wrong number. If your pH ever drifts or crashes, this is the kit that tells you why.

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Both kits together cover the full panel: pH, ammonia, nitrite, nitrate, KH and GH. Test weekly once the pond is established, and daily whenever fish are new, sick or recently medicated.

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.

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.

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.

My pH keeps crashing overnight. What is going on?＋

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.

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