sick goldfish help! asap!

I won my goldfish about 2 months ago at a fair and got him a 5 gallon tank with filter and everything hes been doing great untill the last 2 days

his top fin went down and he looks as tho he got a cut on his side hes also acting weird he used to swim back and forth all day and now he stays more towards the top he also had a weird red mark on his belly almost looks like a cut i just did a 50% water change a few days ago and put treatment in his water like always, ive grown very attached to this fish and I need to know how to make him better

 

how big is this fish, and are there any other fish in the tank?

 

hes still very small maybe 2 inches, and its just him alone.

 

Im sorry to say, I had a fancy tail with a red mark on his belly. He started floating upside down. We lost him I hope you find a cure for this bc I'd also like to know! I was told he had a float bladder issue and there wasn't much to do