My kit says 0.25 ppm. Is that dangerous?
It depends, and not on anything your test kit can tell you. A liquid kit measures total ammonia, but only the un-ionised fraction is really toxic, and that fraction is governed by pH and temperature. Each whole pH unit multiplies it roughly tenfold.
So the same 2 ppm reading can be nearly harmless in one tank and lethal in another. This works out which one you are in.
The toxic fraction at common readings
Freshwater. Find your pH, read across to your temperature, and multiply your kit's total-ammonia reading by that percentage. The result is the free ammonia, and it is the only part of the reading that harms fish. It serves for saltwater too: seawater's salt lowers the free fraction slightly, so for a marine tank these percentages run a little high, which errs on the safe side.
What the bands mean
The bands are free ammonia (NH3), not the total your kit prints. A 2 ppm total reading is Alarm at pH 8 and Low at pH 7, both at 77°F25°C.
How this is calculated
A liquid test kit measures total ammonia nitrogen: ammonium (NH4+) plus free ammonia (NH3) together. Only the free form crosses the gill readily, and the split between them is set by pH and temperature, not by how much ammonia is present.
The fraction is 1 / (1 + 10^(pKa - pH)), with pKa from the Emerson et al. (1975) relationship pKa = 0.09018 + 2729.92 / T, T in kelvin. Each whole pH unit changes the free fraction roughly tenfold, which is why the same reading is harmless in one tank and lethal in another.
The calculator and the table above both use this freshwater relationship, for saltwater as well. Seawater's salt raises pKa by about 0.1 at full strength (Whitfield 1974; Spotte and Adams 1983), so the true free fraction in a marine tank is roughly a fifth lower than the figure shown. That errs on the safe side: it never reports less free ammonia than the seawater relationship would.
What this does not model: nitrite, which is separately toxic and is not measured here; species sensitivity, which varies widely; how long the exposure has already lasted; and the accuracy of your test kit, which for strips is poor. If the fish look distressed, act on the fish, not on the arithmetic.
The other five
Is it big enough?
Floor area or volume, judged against the recognised standards: all of them, when they disagree.
Open tool →Can it eat that?
Staple, occasional, rare treat or never, with the reason. An unlisted food comes back "not on our list", never a guess.
Open tool →What does it really cost?
Setup, monthly, first year and lifetime, itemised, with electricity worked out from real wattage.
Open tool →Which of these two is easier?
Two species on identical axes: space, company, cost, lifespan and daily time.
Open tool →Is this bedding safe?
Safe, caution or avoid for the animal you keep, and why.
Open tool →134 care guides, same data
Every number these tools use comes from the species guides. Read the one for your animal.
Browse the care guides →