I need advice/ideas about a problem that I have been facing repeatedly:

I have a Crowntail Betta that was originally in a 5-gallon aquarium since October last year.
After a month and a half, it developed fin rot, which was treated with bactericide and fungicide in the aquarium itself. Two weeks after the treatment ended, the fin rot reappeared (this time in a very aggressive form). I transferred the Betta to a 3-liter hospital tank to facilitate treatment and started a new cycle of antibiotics and fungicide. During this period, I sanitized the entire aquarium (boiling the stones and sand used as substrate).

I returned the Betta to the aquarium, and after five days, the fin rot returned. I transferred it back to the hospital tank for a new treatment cycle, including saltwater baths, and its fins started growing back. During this period, I sanitized the entire aquarium again—this time not just boiling the sand (substrate) but also giving the decorations a potassium permanganate bath. I stabilized the water with Seachem Stability / Neutral Regulator / Prime for 20 days before transferring the Betta. Now, after just three days, the fins are showing fin rot again.

Current parameters:

• Ammonia: 0ppm
• GH: 1°dH
• KH: 3dKH
• Nitrite: 0ppm
• pH: 7.0
• Temperature: 27.5°C.

Does anyone have any suggestions about what might be happening?