Okay, well I should start by introducing myself. My name is Leah, I am a wife and a mom of 3 children and many.. many animals lol I am new to the board, however I am not new to fish

I have been keeping fish for around 9 years. I have fish that range from Niger Catfish to little corys. Right now I am dealing with a
betta that I recently "adopted" from my college bound sister. She was going to flush the fish to put it out of its misery, however I wasn't exactly fond the idea so I took him home tonight. The fish apparently has chunks of flesh missing from its dorsal area and its fins are completely shredded. She had very rough plastic plants in the tank which I have removed. I infact cut myself on the plants while removing them. I immediately tested the water and surprisingly
ammonia and nitrites are at 0. But none the less, this fish is very sick. It is floating at the top slightly on its side, however it hasn't given up on eating. He will chase after a pellet then go back to laying on it side at the top of the tank. I know Bettas are very sensitive to many medications and I would like to hear personal experiences with a similar situation and what has been used successfully. If anyone could help, please let me know! Thank you!