Aquarium water parameters: what the numbers mean
Ammonia, nitrite, nitrate, pH, KH, GH and temperature: the safe ranges for a freshwater tank, what each reading is telling you, and which ones to act on today.
Seven numbers, three of which matter today
A liquid test kit gives you a row of colours and a card to hold them against, and most of the anxiety in fishkeeping comes from not knowing which of those colours is an emergency and which is a Tuesday. So, before the detail: three of these numbers can kill fish this week, and four are about how well they live.
Act today: ammonia, nitrite, and a temperature that is wrong. Act this month: nitrate, pH, and the two hardness readings, KH and GH. The ranges below are for a freshwater tropical tank, and they are the same ones the app uses to colour a reading green, amber or red.
| Good | Caution | Alert | |
|---|---|---|---|
| Ammonia | 0 | ≤ 0.25 ppm | > 0.25 ppm |
| Nitrite | 0 | ≤ 0.25 ppm | > 0.25 ppm |
| Nitrate | < 20 ppm | 20–40 ppm | > 40 ppm |
| pH | 6.5–7.6 | 6.0–8.2 | < 6.0 · > 8.2 |
| KH | 4–12 dKH | 2–18 dKH | < 2 · > 18 |
| GH | 3–8 dGH | 1–12 dGH | < 1 · > 12 |
| Temperature | 23–27 °C | 20–29 °C | < 20 · > 29 |
| Phosphate | < 1 ppm | 1–2 ppm | > 2 ppm |
Ammonia — the one that burns
Safe: 0. Caution: up to 0.25 ppm. Alert: above that.
Fish excrete ammonia constantly, and in a working tank the filter’s bacteria turn it into nitrite within minutes, so the reading is zero. Any ammonia in a tank with fish means the bacteria are not keeping up: a new tank that has not cycled, a filter that was cleaned too hard or switched off, a dead fish or snail out of sight, a week of overfeeding, or a medication that killed the colony.
Ammonia burns gills. The fish breathe fast, hang at the surface, go red at the gills and dark or pale in the body, and die in days rather than weeks. Its toxicity climbs steeply with pH and temperature, so 0.5 ppm at pH 8 and 28 °C is far more dangerous than the same reading at pH 6.8 and 24 °C. There is a whole guide to the first 24 hours of an ammonia spike; the short version is a 50 % water change now, a detoxifier, and daily testing until it reads zero.
Nitrite — the second act
Safe: 0. Caution: up to 0.25 ppm. Alert: above that.
Nitrite is what ammonia becomes, and a second set of bacteria turns it into nitrate. It appears a few days after an ammonia problem starts, and it is nearly as dangerous: it binds to the fish’s blood and stops it carrying oxygen, so a fish in nitrite gasps in perfectly oxygenated water and its gills turn brown rather than red.
The treatment is the same as for ammonia, water changes and time, with one useful addition: a small dose of aquarium salt, about a teaspoon per 20 litres, blocks nitrite uptake at the gills and buys the bacteria the days they need. Skip the salt with scaleless fish and live plants.
Nitrate — the slow one
Good: under 20 ppm. Caution: up to 40 ppm. Alert: above that.
Nitrate is the end of the chain. It is not acutely toxic, which is why it gets ignored, but fish living above 40 ppm stop growing, breed less, colour down and lose the immune margin that keeps ich and fin rot at bay. Nothing in an ordinary tank removes it except water changes and, slowly, plants.
How fast nitrate climbs between tests is the most useful number the kit gives you, because it is a direct measurement of how much fish your filter and your water changes can actually carry. Climbing 5 to 10 ppm a week on a weekly 25 % change is a tank in balance; climbing 20 or more is a tank with too many mouths or too few water changes. A planted tank that reads under 5 ppm has the opposite problem: the plants are starving, and algae will win.
In the app — every test you log gets a verdict on the spot: which readings are off, by how much, and the water change already sized to your tank’s volume. Ammonia or nitrite in the red caps the tank’s health score at 35 and keeps the home screen red until it clears, so the number you need to act on is never the one you have to go looking for.
pH — stable beats perfect
Good: 6.5 to 7.6. Caution: 6.0 to 8.2. Alert: outside that.
pH is how acidic or alkaline the water is, and most community fish are happy anywhere in the good band. What they are not happy with is change. A fish that lives at 7.8 is fine at 7.8; the same fish moved to 6.8 in an afternoon goes into shock. So the rule is: do not chase a number. Test your tap water, find out where your tank settles, and choose fish that suit it, rather than dosing chemicals to move it.
Two things move pH on their own. A tank that never gets water changes drifts acidic over months, as the buffering minerals are used up; the fix is water changes, not a bottle. And driftwood, leaves and some substrates lower it gently, which soft-water fish enjoy. A pH that swings between morning and evening in a heavily planted tank is normal; a pH that crashes below 6 usually means the KH ran out, which is the next reading.
KH — the buffer that holds pH still
Good: 4 to 12 dKH. Caution: 2 to 18. Alert: outside that.
Carbonate hardness is the water’s ability to resist a pH change. Think of it as the tank’s shock absorber. With a KH of 4 or more, the acids that fish waste and plant decay produce are neutralised and pH holds steady between water changes. With a KH near zero there is nothing to absorb them, and pH can crash overnight, which is what a “pH crash” in a neglected tank actually is.
If your tap water is soft and KH tests low, a small amount of crushed coral in the filter, or a bicarbonate-based buffer dosed slowly, raises it and holds it. If it tests high, leave it alone: a stable pH at 8 is better for almost every fish than an unstable one at 7.
GH — the minerals fish are built from
Good: 3 to 8 dGH. Caution: 1 to 12. Alert: outside that.
General hardness is the calcium and magnesium in the water, which fish and especially shrimp and snails need for bones, scales and shells. It is the reading that decides which fish your water suits: livebearers, rainbowfish and most cichlids want it higher; tetras, rasboras and wild-type bettas want it lower. Soft water with a GH near 1 will give snails pitted shells and shrimp failed moults. Hard water is harmless to almost everything, though soft-water species may not breed in it.
Temperature — the number you set
Good: 23 to 27 °C. Caution: 20 to 29. Alert: outside that.
For a tropical community tank the heater sets this, and the reading only matters when it disagrees with the setting. Too cold and fish stop eating, slow down and become an easy target for ich, which is why an outbreak so often follows a heater failure or a cold night. Too warm and the water holds less oxygen, so fish gasp at the surface even with everything else perfect. A steady 25 °C suits nearly every fish sold as “tropical”; a thermometer separate from the heater is the cheapest insurance in the hobby.
Phosphate — the algae reading
Good: under 1 ppm. Caution: up to 2.
Phosphate comes from food and, in some places, from the tap. Fish do not care about it; algae do. A tank that grows algae faster than you can clean it, with nitrate under control, almost always has high phosphate, and the answer is less food, water changes, and a phosphate-removing resin in the filter for a stubborn case.
In the app — the water lab plots every parameter over time against its good and caution bands, so a nitrate that is creeping or a pH that is sliding shows up as a slope weeks before it shows up as a problem. When the schedule advisor sees nitrate climb 20 ppm across recent tests, it proposes a tighter water-change cadence with the numbers that justify it. Ask Finley to explain these results turns any confusing row of colours into a plain answer about your actual tank.
Common questions
How often should I test my aquarium water?
Weekly, in an established tank, ideally just before the water change so you see nitrate at its peak. Daily in a tank that is cycling, that has fish in it under two months old, that has just been medicated, or whenever ammonia or nitrite reads above zero. Test after every new group of fish for a week, because that is when the bacteria are catching up. Strips are fine for spotting a trend; a liquid kit is what you want for the decisions.
Which is worse, ammonia or nitrite?
Both kill, and both mean the same thing: the filter is not keeping up. Ammonia tends to arrive first and burns gills directly; nitrite follows a few days later and suffocates the fish from the inside. Treat either reading above zero the same way, with a large water change today and daily testing until it clears, and expect nitrite to appear as the ammonia falls.
Should I adjust my pH with chemicals?
Almost never. A bottled pH adjuster moves the number for a day and lets it swing back, and the swing is what harms fish, not the value. If the pH is in the caution band and stable, choose fish that suit it. If it is crashing, the real problem is a KH that ran out, and the fix is water changes and a little carbonate buffer, not an acid or a base.
My tap water tests positive for ammonia. Is something wrong?
Probably chloramine, which many water utilities use instead of chlorine and which a test kit reads as ammonia straight from the tap. A conditioner that handles chloramine neutralises it, and the filter processes the rest within hours, so a small reading right after a water change is normal. Test your tap water once so you know your baseline, and always dose the conditioner for the water you are adding.