A week or so ago I had an algae outbreak in my tank. Not knowing any better I put in algae killer which did the trick. However, my goldfish became very lethargic and stopped eating. I also noticed red streaks near its eyes and tail. After doing some research I realized it had ammonia poisoning so I immediately got it into new water and cleaned out the tank. I don't know if that's the best thing to do but he's doing a lot better now- swimming and eating again. One of his gills was definitely damaged from this- it had what looked like a sore on one side which has now healed but that gill still looks in rough shape. It's ragged and almost looks like it is partially blocked or inflamed. I'm guessing it's most likely not a parasite or bacteria but damage from the ammonia poisoning. Is there anything I should do at this point? Will it be permanently damaged? I feel terrible that I hurt the little guy (who I've had for about 2 years) so any advice is much appreciated!