Ok so I really don't know what I could've done about this death. When I first got him he was in a little cup from Petsmart in blue water with left over food and waste. He looked in perfect condition. Then after about 3-4 weeks his tails curled really badly, and got black at the ends. After about a week of that his tails went almost back to normal with just the very ends bunched together, still black in color. He also had red gills through out those weeks. He also only went to the bathroom once every week if that's normal or not. So in the last few weeks he became nothing but worse. All of his symptoms came back and he would stay in one spot. He also lost appetite quite frequently throughout the weeks. Yesterday his eyes turned into a white blur, and today he was at the bottom of the tank motionless, and covered in goo. I fed him twice daily for the recommended amount, and he never became bloated. He was in a 2 gallon aquarium with a heater that kept the water at 79 degrees. I changed the 75% of the water once a week, siphoning all waste. I added betta water conditioner after every water change. So I don't know what I did wrong.