Need more information please.
Size of tank in gallons?
How many fish and which type?
Water stats in ammonia, nitrite, nitrate, and ph.
Added anything to the tank lately like new fish, plants, changed water conditioner, or used a medication lately.
Check the fish for any of these symptoms.
Darting, erratic swimming.
Laboured breathing, or gasping at surface of tank?
Do any of the fish show signs of excess slime, on body, fins gills?
Do the gills look pale with excess slime, red inflamed?
Do any fish show signs of a grey film on body, a greyish blue film on body?
Do the fish have an opaque body with excess slime?
Do the fish look like there been sprinkled in gold dust, brown velvet patches, yellow golden spots, rusty coloured varnish on fish?
Check fish for red sores, or tiny red pink prick marks?