How do I stop a new betta getting sick?

Two weeks ago I had a snail out break in my tank. I asked on here and many of you were kind enough to give advice. I tried the lettuce leaf and picking them out one by one, but It was getting worse. So I bought two yoyo loaches to help. The larger of the two took a dislike to my betta and had to be removed but he had split one of my battas fins. I hopre good water and time would help heal it but three days ago, cotton wool fungus started to grow on the nipped fin. I treated it straight away. But the next day he was liveless and seemed to of lost the use of the rear fins. wouldn't eat. I knew he had to get to the surfice to breath so I kept coaxing him up but he sank and eventually dround today. I'm so upset! I adore him. He was my first betta. I only had him two months. What did I do wrong and how do I stop it happening again? The tank is 5+ gall, heated, filtered, no sharp edges. 50% water change every week. Good food. I'm really gutted. How do I stop this happening again?