Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
What do you feed the fish.
Do the fish swim fast in circles.
Does the fish look bloated or skinny.
Does the fish look pale or darker in colour.
Are the fish acting listless and lethagic.
Does the fish try to get off the bottom of the tank only to fall back down again.
Can the fish maintain balance.
Does the fish tilt to oneside when resting, or swimming.