New tank syndrome: fish in an uncycled tank, and how to get them through it
Fish added days after the tank was filled are living through the nitrogen cycle. What is happening, the daily routine that keeps them alive for six weeks, and how to know when it is over.
What is happening in the tank
You filled the tank, ran the filter for a day or two as the shop suggested, and added fish. A week later the water is milky, a fish is gasping at the surface, and a test kit, if you have one, reads ammonia. That is new tank syndrome, and it is not a disease: it is the nitrogen cycle happening with the fish already in the water.
Fish produce ammonia constantly. In a mature tank two colonies of bacteria in the filter turn it into nitrite and then into nitrate, and the water reads zero for both toxins. A tank a week old has neither colony yet. They grow only as fast as biology allows, roughly six weeks from empty, and until they do, everything the fish excrete stays in the water with them. There is a guide to growing those colonies without fish, which is the easy way. This guide is for the hard way, because that is the one you are on.
The curve is the whole story. Ammonia rises through week one and two, then falls as the first colony catches up; nitrite rises behind it through week three and four, then falls as the second colony arrives; nitrate appears and the toxins read zero. Your job for six weeks is to keep both peaks below the dashed line, by hand, every day, while the bacteria do the rest.
The daily routine
Test every day. Ammonia and nitrite, every morning, written down. This is not optional and it is not paranoia: the readings tell you whether today needs a water change, and the trend tells you where you are on the curve. A liquid kit, not strips; strips are unreliable at exactly the low readings these decisions turn on.
Change water whenever either toxin reads above 0.25 ppm. Thirty to fifty per cent, dechlorinated, temperature matched to the tank. In week one and two that is most days. Halving the water halves the poison, and it does not slow the cycle: the bacteria live in the filter and the substrate, not in the water you pour out. There is a guide to doing the change safely; the short version is a bucket used for nothing else, a thermometer, and pouring gently.
Dose an ammonia detoxifier between changes. A conditioner that binds ammonia (Seachem Prime, API Ammo Lock and their equivalents) makes it harmless to the fish for a day or two without removing it, so the test kit still shows it and the bacteria still get to eat it. Redose every 48 hours, for the full tank volume.
Feed once every two days, and lightly. Every flake is tomorrow’s ammonia, and the fish are in no danger of starving. Nothing goes in that is not gone in a minute.
Do not touch the filter. Not to clean it, not to change the cartridge the box says to change monthly. It is the only place the bacteria can grow, and rinsing it under the tap in week three puts you back to week one.
Do not add fish. Not one, not to replace one, not because the shop said the tank needs a “cleaner”. Every fish added is more ammonia into a tank that cannot process what it has.
In the app — log the morning test and the verdict tells you whether today is a water-change day, with the change already sized to the tank. The schedule advisor puts a tank with any toxin above zero on a two-day testing cadence and keeps it there until three clean tests are in, then offers to ease back to weekly, so the routine outlasts your memory of it.
Making it shorter
The cycle takes as long as it takes, but there are three honest ways to speed it, and all of them are the same idea: bring bacteria in from somewhere they already live.
- A handful of filter media from a healthy, established tank. A friend’s sponge, a squeeze of their filter floss into yours, a cup of their gravel in a mesh bag. This is the single most effective thing you can do and can cut the six weeks to two.
- A bottled nitrifying culture. The good ones work, the bad ones are water; buy from a shop that refrigerates it and dose the whole bottle on day one. Expect a week or two saved, not a miracle.
- Warmth and oxygen. Keep the tank at 26 to 28 °C if the fish tolerate it and run an air stone: the bacteria grow fastest in warm, well-oxygenated water, and the fish need the oxygen anyway with their gills under attack.
Ammonia-removing resins and zeolite in the filter are the wrong kind of help: they starve the bacteria you are trying to grow and the cycle stalls the day the resin fills up.
Watching the fish
Gasping at the surface, red or brown gills, fast breathing, clamped fins, hiding, darting and flashing against the gravel: these are the signs of ammonia and nitrite damage, and every one of them is a test kit reading you can act on. A fish that is gasping in water that reads zero for both, three days after it last read something, has gill damage and needs clean water, oxygen and time; it usually recovers over a week or two.
Watch for what comes next, because it usually does. Stressed fish with burned gills are exactly the fish that ich and fin rot take hold of, and week three of a fish-in cycle is the classic moment for white spots to appear. The ich guide and the fin rot guide both start with the water for that reason. A fish that dies during the cycle is not a mystery; test, change water, and do not replace it until the numbers say the tank is ready.
In the app — a checkup with a photo of the gills and your readings tells you how much damage the toxins have done and whether a secondary infection has set in, and Ask Finley what to do now walks you through today’s list against your actual tank. The follow-up check-in a few days later asks how the fish are doing.
Knowing it is over
The tank is cycled when ammonia and nitrite both read zero every morning for a week, with the fish eating normally, and nitrate is rising. That last part matters: nitrate is the waste product of the whole chain, and its presence is what proves the conversion is running rather than the ammonia simply being diluted by your water changes.
Then, and only then, the routine relaxes: a weekly test, a weekly 25 % water change, feeding every day. Add the next fish a few at a time, two weeks apart, and test for a week after each group, because the colonies are exactly as big as the fish they have been feeding on. How many fish the tank can carry is the next question, and the test kit answers it too.
Common questions
How long does new tank syndrome last?
About six weeks from the day the tank was filled, whatever you do, and two to four with seeded filter media from an established tank. The ammonia phase usually peaks in the second week and the nitrite phase in the fourth. A tank that still reads toxins in week eight is being reset: check for a filter that was cleaned, a medication, or chlorinated top-ups.
Should I do a water change every day?
Only when a reading says so, which in the first fortnight is most days. Change 30 to 50 % whenever ammonia or nitrite reads above 0.25 ppm, and leave the water alone when both read under that, because the bacteria need some ammonia to grow on. Daily testing is what decides; daily changing by rote slows the cycle without helping the fish.
Can fish survive new tank syndrome?
Most do, with the routine above. Hardy species, a lightly stocked tank, daily testing and a detoxifier between changes get the great majority through. Losses come from tanks that were fully stocked on day one, overfed, or not tested, and from delicate species that should never have been the first fish. A tank that has lost fish is not a failed tank; test, change water, and finish the cycle before adding any more.
Will a bacteria supplement fix it?
It helps and it does not fix it. A good refrigerated nitrifying culture can save a week or two; filter media from a healthy tank can save four. Neither replaces daily testing and water changes, and nothing makes a tank safe the day fish go in. Be suspicious of any product that claims to.