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Probably more than 90% of diseases in captive fish can be prevented by avoiding stress. Stress weakens fishes' immune systems, leading to increased susceptibility to disease. Actually, diseases and pathogens are almost always present in tanks, but a healthy fish's immune system will prevent them from being a problem. Some of the most common stressors for captive fish are:
Poor water quality: measurable ammonia or nitrites, or very high nitrates.
The water temperature is fluctuating more than 2 ° C/day
Incompatible species in the tank.
Too many fish in the tank .
The tank is too small for the fish.
The water is too warm or too cold for the species.
wrong pH for species.
pH fluctuations greater than 0.2 units/day.
Insufficient cover or hiding places present.
Wrong water hardness for the species.
Insufficient oxygen in the water.
Improper fish nutrition (wrong food, foods not varied).
Never buy sick fish from a store. Especially do not buy fish or plants from a tank if *any* fish in the tank shows any signs of disease or if there is medicine in the water (water is colored yellow, green, or blue). Store people may say the fish are fine, but if they were, why is the medicine in the tank? Also ask how long the fish have been in the store. New arrivals may be carrying diseases that have not shown up yet. It is better to wait a couple of weeks before purchasing the fish. If you must have a fish that just came in, be especially sure to quarantine it properly.
Most important: watch your fish and know what their normal behavior and appearance is. If you don't know what normal is, you can't know what sick is.
Clamped fins (fins are held abnormally close to body)
The fish refuses its usual food for more than 2 days.
There are visible spots, lesions, or white patches on the fish.
The fish gasps at the surface of the water.
The fish floats, sinks, whirls, or swims sideways.
The fish shimmies (moves from side to side without going forward).
A normally active fish is still.
A normally still fish is very active.
The fish suddenly bloats up, and it's not due to eggs or young.
The fish is scratching against tank decorations.