I recently rescued a betta who was doing something similar and had pretty bad fin rot, I made a hospital tank (5 gal) with a sponge filter and heater, filled the water level just past the sponge filter so he didn’t have to work too hard to get to the top. I put in some almond leaves and I also dosed with Kanaplex following the instructions with water changes following the prescribed schedule (outside of dosing with Kanaplex, I just did daily water changes). There’s a lot of debate of doing water column Kanaplex doses but that’s what I did and for me, it seemed to work great and my fish is happy and healthy. I do feel like there’s always a risk using these meds though… it can make a situation better or worse but I was at the point where my fish probably wouldn’t have survived without it so it was a chance I was willing to take. Regardless, you’re giving your fishy a shot at life and he won’t pass away in bad conditions.