Fancy Goldfish

I went on vacation last march and my fish was overfed and I wasn't checking the water and he started getting red streaks on tail and fins. This happened 3 times before I got everything under control. His water's fine, I have turned the heater down from 23° to 22° this morning hoping that he prefers colder water. I was only giving him 1 pellet in the morning and another at night. I gave him 2 this morning hoping he was stressed because he was hungry. There are no other fish in the tank. I changed 30% of the water this morning and I gave him a salt dip for 9 minutes this morning. I noticed he gets red streaks when he poops and then an hour later they're gone. This has been going on for about 7 months now and I have run completely out of ideas. I have no clue what to do.