help with betta infection and injury care

[rivers]

I recently set up a nano aquarium, a 2.1-gallon tank, with no intention of keeping fish, just a planted tank and maybe some dwarf shrimp. However, after getting it cycled, I found this male roundtail koi betta, labeled as a "galaxy" betta. He was tiny compared to other bettas I've seen, so I thought maybe he'd work out, with the plan of upgrading to an 8-gallon long tank ASAP. What I didn't notice initially was the lump on the side of his head - it looked like a tumor or cyst behind his gill.

The first few days were fine; I didn't feed him the first day to avoid stress, and he picked at his pellets the next two days. But on the fourth day, he took a turn for the worse. I came home from work to find the lump had burst, leaving a gaping hole in the side of his head. I thought he was a goner, but he swam up to the surface for air, and I decided to leave him be for the night, expecting the worst by morning. However, he was still alive the next day, and the day after that. It's been over a week since the hole appeared, and it's slowly started to close up.

I didn't want to waste money on medicine for a fish I thought would be dead soon, so I didn't do anything special until now, except add a tannin booster with some "soothing acids" daily. Yesterday, I started him on antibiotic and anti-fungal treatment, and he's eaten freeze-dried bloodworms for two days in a row. The wound is still there, but it's much smaller than before.

My water parameters seem okay - temp is around 78-82, and I do frequent water changes (5-15% every three days). I've attached some images of Gossamer.

My question is, does anyone know what could have caused this, and am I helping him correctly? I've been unable to find any info on this kind of illness. Also, considering he's not out of the woods yet, is it safe for him to be healing in a 2-gallon tank? I'm hesitant to move him due to stress and potential shock risks. Any tips or advice on helping Gossamer would be greatly appreciated.

[kingrise]

Gossamer is really something else, I have to say. I was pretty concerned when I first saw the lump on his head, and to be honest, I didn't think he'd make it. Tumors or cysts usually don't end well for fish, I've heard. I'm not trying to be a downer, but I think rivers might appreciate some expert advice from someone like @coltin - they might have a better idea of what's going on and how to help Gossamer recover.

[Snout]

Your betta's a stunning creature. A bigger tank would definitely be beneficial for him - the parameters tend to be more stable and a filter would help keep the water clearer. Plus, you could do bigger water changes. I'd recommend going for 50% water changes to keep the water really clean.

[Snout]

I'd be happy to provide more photos of Gossamer's wound for reference, as I actually already attached a few to my original post, but I'll try to get a clearer one of the affected area if you'd like.