Nitrogen cycle checker: which stage is your tank in?
Your ammonia, nitrite and nitrate readings over time — which stage of the cycle the series puts your tank in, and what to do next.
Add your first test above. One reading tells you very little; the third one starts telling you where the tank is heading.
A single test result is a colour on a card. The same reading means opposite things depending on what came before it: nitrite at 2 ppm with ammonia still climbing is a tank in the middle of its cycle; nitrite at 2 ppm that read 5 ppm three days ago, ammonia already at zero, is a tank at the end of one. At the end for the filter, at least: 2 ppm nitrite does the same at the gills in both. So this page asks for a series, not one set of boxes: the order the three tests moved in is the answer. That order holds in almost every new tank — ammonia climbs, the bacteria that eat it arrive, nitrite climbs, the bacteria that eat nitrite arrive, and nitrate is what is left. Seeded media can flatten either hump, so a tank that never spiked is not a tank that lied to you. Nitrate is the lasting evidence that the work was done, which is why nitrate that came in your tap water earns no credit, and why a box you left untested stays unknown rather than becoming a zero. pH and temperature set how much of your total ammonia is free ammonia, the fraction that burns gills: the same 0.25 ppm is nothing at pH 6.5 and worth acting on at pH 8.5. Nothing here reads the clock; how long something has been going on is measured between your own readings. Whether fish are in there changes every instruction that follows. Empty, ammonia at 3 ppm is the point of the exercise and the answer is to wait, keep the heater up and leave the filter alone. With fish, it is something they are breathing, and the answer starts with getting it out of the water: half the tank, dechlorinated and at tank temperature, today. A fish-in cycle is a welfare problem being managed, not a method worth choosing.
Common questions
- How long does it take to cycle a new aquarium?
- Four to eight weeks is usual at tropical temperature, but the calendar is not what tells you when it is finished. Ammonia-eating bacteria normally appear within two weeks, so ammonia sitting there for a fortnight with nitrite never showing is a stall with a cause. The finish line is a full dose of ammonia cleared to zero, nitrite at zero beside it, inside 24 hours — two zeros in a tank nothing is feeding mean nothing has started. Then add fish a few at a time.
- How can I speed up the nitrogen cycle?
- Warmth does more than anything else you can do — 28 °C runs the cycle about half again as fast as 22 °C, and with no fish in there nothing minds it. Keep KH above about 3 °dKH so the pH does not fall as the cycle acidifies; under pH 6.5 nitrification crawls. Do not overdose ammonia: past roughly 5 ppm the free fraction stalls the nitrite-eaters, the half of the pair that gives out first. Hold 2–3 ppm, lower above pH 8. Mature media is the only real shortcut.
- Do I need fish to cycle a tank?
- No, and it goes better without them. Ammonia dosed to 2–3 ppm, or a pinch of food left to rot, feeds the filter while nothing is suffering. It has to be the plain solution with nothing else in it — household cleaning ammonia is usually cut with surfactants or perfume, and those stay in the tank long after the cycle is over. An empty tank also lets you push what speeds it up: 28 °C, no water changes, ammonia topped back up.
- Why is my nitrite not dropping?
- The nitrite-eaters are the slower half of the pair and often need one to three weeks more after ammonia clears. Three things stall them: pH falling under 6.5, which is a KH problem before it is a pH one; media rinsed under the tap, where chlorine kills what you were waiting for; and a nitrite load from ammonia dosed far too high. With fish in there, plain aquarium salt at about 1 g per litre lets chloride take nitrite's place at the gills and buys you the days you need.
- Is bottled bacteria worth it?
- Sometimes, and never as a replacement for testing. What is in the bottle matters more than how it was stored: the nitrite-eater that textbooks credited for decades barely establishes in an aquarium filter, so products built on that strain do very little, and weeks on a warm shop shelf are a second gamble you cannot see. A squeeze of mature sponge from a healthy tank is the right bacteria, free, and already at filter density.
- How do I cycle a tank with fish already in it?
- Test ammonia and nitrite daily. Whenever either is above about 0.25 ppm, change roughly half the tank, dechlorinated and at tank temperature — untreated tap water kills the filter bacteria and burns already-irritated gills. Use a dechlorinator that binds ammonia, redosed every day or two; the bound form still shows on a test kit. Add salt once nitrite appears, dosed for the water you replace, not the whole tank again. Feed lightly, add no fish, and leave the filter alone unless flow drops, then rinse it in old tank water.