My yellow double tail was acting weird a week after I got him. He started hiding behind the filter and only come out when I fed him. I noticed he was having a swim bladder issue lying on his side. If I put my eye dropper in the water and sort followed him he would swim around fast and get tired and float on his side. Well I stopped feeding him for 3 days to see if it would resolve but it didn't.
I decided to take him out of his tank and put him in a glass jar about half gallon in size. I added salt, 4 to 5 pieces of rock salt. I put some ammolock to grab any ammonia that would be produced and also added some black water extract to help him feel at ease and heal.
I went to the store and bought him some brine shrimp to eat and some medicated pellets. He tried to eat but would spit his food out, even the live brine shrimp. I then noticed something in the bottom of the tank which was yellow the same color as his fin. I was thinking that maybe one of his fins broke off but they all seemed intact. I had a small microscope and put the object under it. I discovered two different surfaces when I flipped it over. I believe I had some sort of leech, or fluke that had been living in his gills. After it fell off he started perking up, the next day he was eating and no longer was floating on his side. The follow day I woke up and found him making bubble nest and swimming near the middle of the tank flaring at the males in the next tank.
Well my point is that it's not always swim bladder disease, it can be parasites. Also I found some small round things in the bottom of the tank, I'm thinking those might be eggs as his belly was quite swollen. Today I just changed his water and added the same regiment to it. He is eating the medicated food and brine shrimp and is doing well. I want to make sure all the parasites are dead so I'm continuing the salt treatment.
Good luck hope this might be of some help to someone.