I would really appreciate any expertise/advice.
My betta, Fishface, has a persistent hole in his tail. Multiple internet searches have yielded nothing. The closest things I could come up with were fin rot (which I don't think it is, because the affected tissue is pale instead of blackish, it started in the middle of the fin, and there is no tattering of the fins, and it has persisted at a low level for months), or a herpes-like virus (which I also don't think it is, because I've never heard of those making holes and blisters, but more like lumpy tumors).
You can see the hole clearly in the first pic below, with the wood in the background showing through his dark fin in what looks like a slash mark. What you can't see clearly is that the hole isn't ragged, and in fact is surrounded by a continuous border of blisters of varying sizes, with one large blister on the edge of the hole closer to the fish's body. The blisters are slightly discolored. Mostly pale, but some mottled darker areas too.
Here's the details:
Symptoms and Treatment
Appearance: A hole in the middle of his tailfin, surrounded by what looks like blistering. Slowly getting bigger. No tattered, shredded, or torn looking fins. Just the one clean hole that surrounded by by a raised discolored area.
Behavior: Behavior and apetite unchanged. Appears normal.
History if symptoms: The thing started as a pale spot in the middle of his tail fin that was there for months. Then it turned into a blister which stayed for about a month, and now theres a lesion, surrounded by what looks like a ring of blister. Only the one spot is affected. Its getting bigger, but slowly.
Treatments: extra stress coat.
Health history: Had swim bladder sickness once, and had Ich once. He recovered from both fine. Had some minor spinal and gill malformations when I got him from the pet store. Spine is slightly S-shaped, and one gill can't close fully.
Age: 3 years.
Tank size: 10 gallons, but only half filled, so 5 gallons effective, 2nd photo.
Temperature: between 75 and 80 F (depending on the weather)
Filter: yes, powered filter/fountain full of gravel and moss, custom made (by me), 3rd photo.
Aeration: yes, small fountain.
Tankmates: lots of snails, 3 small shrimp, 1 very large (nonparasitic) leech.
Substrate: gravel, large rocks, 1 freshwater mussel shell (to help keep the snail's shells healthy), waterlogged wood (helps acidify the very basic tapwater), and lots of plants.
Food: Betta bites, sometimes a bit of frozen pea
Feeding: 1 to 2 times daily
Water changes: weekly
Percent change: 1/3
Additives: API stress coat, or API tap water conditioner
Ammonia: (not included on my test strips)
Sorry for the low quality pics!