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 leaning to one side when resting?
Does the fish look bloated, or thin?
Do the gills look red and inflamed, pale gills with excess slime?
Does the fish dart around the dart, or signs of erratic swimming, turning up side down?
Can the fish maintain balance in the water?
Is the fish acting listless, and lethargic?
Any signs of red streaking of the fins, or body?
Any signs of your fish flicking and rubbing on objects in the tank?
What does it look like when your fish go to the toilet?
Sorry for all the questions.