Unstoppable Fin Rot?

Hello!

My wife and I set up our first tank a little over a year ago and in doing so I spent quite a while on these boards learning. I'm so glad they are here and so many are willing to help.

In addition to our 10 gal aquarium, we have a small(er) 2.5 gallon in which we have a single betta and japanese algae eating shrimp. And our poor betta has been ill for some time with a small whitish looking area where his dorsal fin meets his body.

My first thought was fin/tail rot, so we have treated (not consecutively, and not necessarily in this order) with Jungle Labs LifeGuard, Mardel Maracyn (erythromycin), API Tetracyline, and Mardel Maroxy (for fungus, did treat consecutively with other medication). All to no luck. I've added freshwater aquarium salt to the water, and have done multiple water changes (I change his water every 2 weeks regularly, and the tank is filtered with the filter set to the lowest setting - ammonia/nitrites always at 0, nitrites may become detectible right before a water change).

As the issue was where the fins met the body, not starting at the edge of the fins and moving toward the body, I was hesitant to think fin/tail rot... but not knowing what else it might be, that's what I attempted to treat. Additionally, his fins looked as if they might be getting shorter, so I thought it would be prudent to treat for fin/tail rot anyway - and hope the other went away.

Now I'm at a loss for what to do for the little guy and was hoping someone might recognize it as some other malady, or possibly be able to give a better/stronger treatment if it is fin/tail rot.

Thanks so much for the time!

Thanks for the reply. I knew that bettas without filtration needed a water change every several days (edited from "weeks"), but had thought with the filtration I might not need to do them as often. I also read, before buying him, various arguments for/against filtration for bettas. I opted for filtration, with the filter turned down to barely moving any water.

When we first got him, I checked ammonia/nitrite/nitrate levels frequently until, after two weeks, I wanted to do a water change. Then kept watching for a while afterwards. As the levels either were never detectable, or barely detectable right before a water change, I assumed the water was fine for him.

We do have a heater in the water, and it stays between 78 and 82/83 in his tank. As for food, he eats HBH Betta Bites, and once or twice a week gets frozen bloodworms (thawed). We tried alternating the Betta Bites and a flake food for bettas (I don't recall the name), but he wouldn't eat the flake food.

There isn't anything terribly sharp in his tank. There is a silk plant he likes to sit on, a plastic plant that doesn't have sharp edges (more rubbery than hard plastic). And there is a piece of wood which algae grows on for the shrimp to eat (not very much). He does like to hide behind the wood, as it is propped against a corner of the tank, so it is possible he is scraping himself on it. The wood is very soft, but for his fins it may still be "sharp."

Okay, more frequent water changes it is. As for the medications, those I listed were over the course of several months. Sorry if I gave the wrong impression. We would try one, then wait two to three weeks, watching for improvement. If none, we'd move on to something else.

Thanks to you all for all the input. I really do appreciate it! And thanks for the welcome. I wish I had joined earlier so my first post would have been a "Hi, this is me" rather than a "OH NOES!"