New tank syndrome: what it is and how to fix it

New tank syndrome is ammonia and nitrite poisoning in an aquarium whose filter has not yet grown its bacterial colony. The fish gasp at the surface, clamp their fins, stop eating and die in sequence over the first weeks. It is not an infection and no medication treats it. The fix is water changes, an ammonia-detoxifying conditioner, sparing feeding, and finishing the cycle.

The name makes it sound like a disease, which is unhelpful, because it sends people to the fish-medication shelf for a water-chemistry problem. Nothing on that shelf helps and some of it makes things worse by killing bacteria.

What is happening

A new filter is sterile. It contains none of the bacteria that convert ammonia to nitrite and nitrite to nitrate, so on day one it processes no ammonia at all. Fish added to that tank are producing ammonia continuously into water with nothing to remove it.

Ammonia climbs over days. It burns gill tissue at concentrations well under 1 ppm, and — the EPA documents this — its toxicity rises with both pH and temperature, so a warm tropical tank is the worst case. Damaged gills cannot take up oxygen properly, which is why the fish gasp at the surface in water that is perfectly well oxygenated.

Then it gets worse before it gets better. Around week two the first bacteria establish and ammonia starts falling — but they convert it to nitrite, which is also toxic, interfering with the blood's ability to carry oxygen. Many keepers see ammonia dropping, relax, and lose fish to the nitrite spike that follows. There are two poisonings, not one.

Recognizing it

The emergency protocol

If you have fish in an uncycled tank right now, do this. Do not tear the tank down, do not medicate, and do not add more fish.

1. Test ammonia and nitrite today, and every day from now on. You are managing a number.
2. Change 25 to 50% of the water whenever either is detectable. Dechlorinated, temperature-matched. This is the entire treatment and there is no substitute for it.
3. Dose a conditioner that detoxifies ammonia. It converts ammonia to a less toxic form the bacteria can still consume, buying time between changes. It does not remove ammonia and your test will still read it. It is not a substitute for the water change.
4. Stop feeding for two days, then feed sparingly. Every uneaten flake becomes ammonia. Fish tolerate light feeding far better than they tolerate ammonia — a healthy fish is fine for a week without food.
5. Add bacteria. Bottled starter, or far better, media from an established tank. See do bacteria starters work.
6. Do not change the filter media, and do not clean the filter thoroughly. Whatever colony exists is in there.
7. Do not add fish, and do not move the fish to a bucket — an unfiltered bucket is worse.
8. Keep going until ammonia and nitrite both hold at zero. Weeks, not days.

What does not help

• Fish medication. This is not an infection. Antibacterial medications actively make it worse by killing the nitrifying bacteria you are trying to grow.
• Ammonia-removing chemical media. It strips the ammonia the colony needs to feed on, so the tank never finishes cycling and you stay dependent on the media.
• Adding more fish. More ammonia into a system that already cannot process what it has.
• Complete water changes. A 100% change strips the colony's food supply and shocks the fish. Frequent partial changes are better.
• Cleaning the filter thoroughly. The worst possible timing — the colony lives in there.
• Waiting for the water to clear. Clarity is unrelated to ammonia. See why is my fish tank water cloudy.
• Aquarium salt. Sometimes suggested for nitrite specifically, and while chloride does compete with nitrite uptake, it is a hedge rather than a fix and many soft-water species dislike it. Water changes remain the answer.

How long this lasts

A fish-in cycle typically runs six to eight weeks, longer than a fishless one. That is not bad luck: your water changes keep ammonia low, which is exactly right for the fish and means the bacteria are being fed less, so the colony grows more slowly. You are deliberately trading speed for survival.

The sequence is the same one a fishless cycle goes through, just stretched: ammonia first, then a long nitrite plateau, then nitrate. Full explanation on how to cycle a fish tank.

Preventing it entirely

Cycle the tank before the fish. That is the whole of it, and it is why this site is organized the way it is.

• Do a fishless cycle — four to six weeks with nothing at risk. Fishless cycle step by step.
• Seed the filter with media from an established tank and it can be under two weeks.
• Buy the test kit with the tank, not after the first fish dies. Best aquarium test kits.
• Stock slowly even after cycling, because the colony sizes itself to the load it has been fed. Adding fish to a new tank.
• Ignore the shop assistant who says a day is enough. It is not, and it is the most commonly repeated bad advice in the hobby.