What your aquarium test results mean
Type your test kit’s readings over the example. Leave any you did not test blank. You get what each reading means for your tank and what to do today.
For the example: 2 to watch (nitrate 30 ppm, KH 4 dKH). What to do ↓
What to do today
Worked example for the example readings
2 to watch
WatchNitrate 30 ppm: Nitrate creeping up
Between 20 and 40 ppm. Change a little more water each week, or change it more often, to bring it under 20.
WatchKH 4 dKH: Low KH
About 71 ppm as CaCO3, under the 100 ppm (5.6 dKH) that keeps the filter bacteria working well. pH can drift down between water changes; test it more often.
GoodAmmonia 0 ppm: No ammonia
This is where a tank with fish should be.
GoodNitrite 0 ppm: No nitrite
Good: the bacteria that turn nitrite into nitrate are keeping up.
GoodpH 7.2: pH in the usual range
A steady pH matters more than a perfect number. Compare it with the range on each of your fishes’ care pages.
GoodGH 8 dGH: Medium hardness
Suits most community fish, shrimp and snails.
Next: identify algae · sick fish · work out a water change · cycling guide · which test kit measures what
How to read the results
Each test in detail: ammonia · nitrite · nitrate · pH · GH · KH.
Ammonia and nitrite should read 0 in any tank with fish. Both come from fish waste and food, and both harm fish at low levels: nitrite from about 0.1 ppm, according to the University of Florida’s guide to ammonia (UF/IFAS FA031). A water change is the first fix, with one exception: when ammonia shows up in acid water, below about pH 6.5, change water a little each day instead, because a big change raises the pH and turns more of the ammonia toxic (Merck Veterinary Manual).
Nitrate is the end of the nitrogen cycle; it builds up between water changes and is kept down by changing water and by plants. The 20 and 40 ppm marks here are common hobby targets for a community tank, well below the levels that poison most fish; sensitive species do best at the low end.
A new tank with fish has no finished filter yet: the bacteria can take up to 8 weeks to establish. Zero readings that day are good news, not proof the cycle is done, so keep testing ammonia and nitrite every few days. Adding ammonia to grow bacteria is only for a tank without fish.
pH is how acidic the water is; KH is the buffer that holds pH steady; GH is the calcium and magnesium that fish, shrimp and snails need. Merck advises keeping KH at 100 ppm as CaCO3 (5.6 dKH) or more, because at 50 ppm (2.8 dKH) or less the filter bacteria slow down and the pH can crash. The pH and GH notes describe the usual ranges of the fish in our guide; each species has its own, so compare with its care page.
Free ammonia is the toxic part (NH3) of a total-ammonia reading. Its share rises with pH and temperature, which is why the same reading is far more dangerous at pH 8 than at pH 7; UF/IFAS warns that above 0.05 ppm free ammonia damages fish tissue. We calculate it with the standard formula of Emerson and colleagues (1975), the one behind the published pKa of ammonia of about 9.25 at 77 °F (Wikipedia: Ammonia).
Fish gasping at the surface, lying on the bottom or showing red gills need a water change straight away, whatever the kit says. For disease, a vet who treats fish can diagnose it and advise treatment.