its hard coz you have to make the decisions yourself as he cant tell you anythin.
maybe you cud try again with his fish and see how he is give it a few days then try him again. its abit like humans like if we drink loads get a hangover and then do it all again or i no when i eat indian curry it kills me the next day but its a sacrifice i like to take lol.
i no its not really the same but its similar same principles.

you cud maybe give him the food the vet recomends and then give him his fish as a treat. it cud be that his old diet was jst too much so you cud cut that down mix in the special food and everyones happy.

you no the signs so you can call the shots if anythin happens even if its a minor symptom you can jst stop it till it clears.