Need size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia,nitrite,nitrate, and ph.
Has the fish gone pale or darkened in colour.
Does the fish look bloated.
What does it look like when the fish goes to the toilet.
What do you feed the fish.
Laying to one side can be bacterial to parasite, so check for signs of flicking and rubbing against objects, and also check the anus to see if its enlarged or red and inflamed.