The flow rate is 90 GPH, the tank is 10 gallons
When his fins were deteriorating there was a black edge, however, I haven't seen it in a while. His tail appears to be getting better now, but the small cut on his lower fin is getting worse (but no black edges).
Also worth mentioning - he used to have one of those betta hammocks in there with him. After reading the horror stories on this site I decided to examine it more carefully. The bottom had a tiny bit of rust on it. I removed it - not sure if this may have contributed to the issue.