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 your fish?
Isolate the fish if the other fish in the tank are pecking at her.
The mark on the head does it look bleached out?
Does the mark resemble a saddle shape. Going along the back, and down the sides of the fish to form a saddle shape.
Does the mark look like cotton wool, or patches of cotton wool?
What colour is the mark?
Any redness to the mark?
Is the mark greyish white with a circling of dead white tissue around the edges, or a red centre.
Is the skin peeling away?
Can the fish maintain balance in the water?
Are scales sticking out?
