Extreme Case of Fin Rot - Suggestions please!

[Sagat]

Yeah, they look awfully clear to be fin rot. How much motion does the filter put in the water? I was told they'll bite their fins down if they get annoyed at how much resistance their tail is giving them.

[Beverllee]

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.

[Sagat]

Wow. Both my tanks have their filters set to the lowest setting, and have a DIY filter baffle to reduce flow even more.

I'd probably watch him swim and see if the water flow bothers him or if he prefers (or avoids) certain areas of the tank over others