Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
Does the fish look pale or darker in colour.
Is the fish acting listless and lethagic.
Does the fish try to get off the bottom of the tank only to fall back down again.
Is the fish leaning to oneside.
Any signs of flicking and rubbing, excess mucas, laboured breathing, gasping, darting, or erratic swimming.
Does the fish look bloated or skinny.
What does it look like when the fish goes to the toilet.
Any sores or red streaking on the fish body.