New tank syndrome
Treat nowWater problemAlso called: Ammonia poisoning, Nitrite poisoning, Uncycled tank
What you see
In a young tank fish clamp their fins, hide, refuse food, gasp at the surface, gills redden, dark patches can appear on the body and fins fray. It looks like disease — but most often it is ammonia or nitrite poisoning in a tank whose filter has not grown its bacteria yet.
How to tell it from its look-alikes
What separates this from real disease is not the eye but the test: measure ammonia and nitrite. If either is above zero, you have a water problem first — whether or not a disease sits next to it. Nearly every poisoning sign (clamped fins, lethargy, gasping, red gills) overlaps with the signs of most diseases, so guessing from appearance is impossible in principle here.
The three common causes leave different tracks. Tankmate nipping — clean, scissor-like notches with no eroding rim, and it continues only while the nipper is present (the planner warns which species nip fins). Bacterial rot — an uneven, crumbling edge with a rim, usually in poor water. And fins going together with slow wasting of the fish points to mycobacteriosis, which likes to impersonate fin rot. Which cause is yours is settled most reliably not by how the fins look but by a water test.
It differs from ich in shape: not discrete round grains but flat, spreading patches. From true fungus it differs three ways: the columnaris film lies flat rather than tufting upward, is often yellowish rather than grey-white, and spreads faster — visibly within a day or two, where fungus takes several days. Only a microscope separates these two for certain — so when in doubt, act as if it were columnaris, the faster and deadlier of the pair.
The separator from ich is grain size: velvet is dust-fine with a gold or rust tint, while ich forms discrete white grains. Rubbing (flashing) is common to both, so it separates nothing. When unsure, use a flashlight in a dark room — a dust sheen glitters, grains stay as dots.
The dangerous mistake
Buying medicine for a “disease” that is not there. No medicine removes ammonia — and several medicines also kill the very filter bacteria the tank is missing most, so the “treatment” deepens the poisoning. The second mistake is rinsing the filter under the tap: that washes the bacteria away and restarts the cycle from zero.
Water first
In a young tank, most “disease” is not disease — it is ammonia or nitrite poisoning, and no medicine treats that. Before buying anything, test ammonia, nitrite and nitrate. If you keep the cycle log, the answer is already there.
What to do
Change 30–50% of the water immediately (prepared, dechlorinated) and repeat daily until ammonia and nitrite hold at zero. Feed lightly or skip a day, and do not run the tank warmer than the species need. Keep measuring daily — the AquaSage cycle log exists for exactly this: it shows whether the cycle is moving and tells you when it is safe to add more fish.
What to expect
With the water fixed, fish usually recover within days, though badly burned gills leave lasting sensitivity. The cycle itself completes within weeks — and this is the one “disease” cured with nothing but a bucket and patience.
This reference helps you recognise and tell conditions apart — it does not make the diagnosis. Only a microscope or a veterinarian confirms one. Use medicines exactly per the label, ideally in a quarantine tank, because several of them kill filter bacteria.