Just want to thank Sena and everyone else on this board for your help. Sushi is doing much better and almost looks like his old, perky self. His fins even look much better.
Just FYI... I stopped using the BettaFix immediately after reading that it can cause fin melt. I started using Stress Coat (water conditioner and also protects the slime coat) and Maracyn-Two (antibiotic). While the water was very cloudy (side effect of the Maracyn-Two), it did help a lot. I also added larger rocks and created several caves for Sushi to hid in, which he LOVES! While I have had other bettas live several years, I think I was just lucky. It is crucial to have a heater... which I never knew. I hope that Sushi continues to heal and lives a long, happy life. I know he would not be here today without the help of this forum. Thanks again!!!