My betta came up with fin rot about a month ago. I tested my water and Ammonia 0, Nitrite 0 Nitrates less than 5ppm. I did a good vacuum and 20% pwc. I did 20% every second day for a week but the rot continued. LFS sold me Methylene Blue | Kordon and said to move him to a hospital tank and treat him with the in-tank dosage as per the package instructions which I did. I completed the full 5 days and he seemed better. I left him in his hospital for 2 more days and he seemed well on the mend. I put him back in his tank which still tested with all good parameters. Two days later the fin rot was back and much more aggressive. Back into the hospital tank with an almond leaf and 80% daily waters changes (his hospital is only 1G). This seemed to slow it down but it didn't stop it. LFS said to repeat the methylene treatment which I am day 3 of but it isn't working and the illness is progressing much faster now and the betta is looking weak and ill. He's still and doesn't come to say hi when I get close. The LFS can now told me to "make him comfortable because he's probably just old".
The fish was fine a month ago. That sounds like a crap answer from someone who doesn't know what they're talking about. Is it really too late or should I try something else?