Zero means zero
Ammonia and nitrite are the two toxic middle stages of the nitrogen cycle, and a working filter processes both as fast as the fish produce them. That is why, in a genuinely cycled tank, a liquid test should find neither — the reading is not 'low' but zero, test after test. This surprises keepers who expect a small background level, as with nitrate, but the biology allows none: whatever ammonia appears is a meal for a bacterial colony sized to eat it, and the same holds a step later for nitrite. Any detectable reading means production has outrun the colony, or the colony has shrunk.
Nitrate: the exception that proves the rule
Nitrate, the cycle's end product, is meant to register. Fish tolerate it at the modest levels a maintained tank carries, and it leaves only through water changes and plant growth. A healthy set of results therefore reads: ammonia zero, nitrite zero, nitrate present but controlled by your change routine. If nitrate reads zero too in a stocked tank, suspect the test before celebrating — an unshaken reagent bottle in the nitrate test is a well-known source of false zeros.
What a sudden reading is telling you
In an established tank, ammonia or nitrite appearing after months of zeros is a message with a short list of likely senders. A fish has died unnoticed and is decaying in the plants. The tank is overfed, or newly stocked beyond the colony's size. The filter was switched off too long, its media was replaced all at once or rinsed under the tap, killing the bacteria it housed. Or a treatment used in the tank damaged the colony along with its target. The reading itself tells you the filter is behind; recent history tells you why.
What to do when zero becomes a number
Dilute first: change a portion of the water with dechlorinated, temperature-matched water to bring the level down, and repeat daily while readings persist. Feed lightly or skip a day — input sets the colony's workload. Search the tank for the cause: count the fish, check the filter is running at full flow, review any media changes. Bottled bacterial supplements can support recovery, but the water changes are what keep fish safe while the colony rebuilds over the following days.
How often to test a settled tank
A mature, stable tank needs only occasional confirmation — many keepers test monthly, after any change to stocking or filtration, and immediately whenever fish behave oddly. The value of those routine zeros is the baseline: you cannot recognise an abnormal reading in a tank whose normal you never recorded. Test after holidays and house-sitters, too; overfeeding by kind visitors is one of the commonest causes of a surprise spike. Keep the test kit's age in mind as well: ageing reagents drift, and a surprising reading from an old kit deserves a repeat with a fresh one before you act on it.
The pattern only becomes visible across time, which is why logging every result in your tank's Pet Passport on KinsLe — zeros included — gives you the one thing a single test cannot: proof of what your filter usually achieves, and the earliest possible sight of the day it stops.