Please help! I have followed some guidelines for problem statement - I hope this helps you help my fish!
1. Size of your tank? 5.5 gal
2. Water parameters? State the brand of test kit used. 0 ammonia, 0 nitrite, < 20 nitrate. Jungle test strips used
3. Aquarium set up? freshwater
4. How long the aquarium has been set up? 6 months
5. What fish do you have? How many are in your tank? How big are they? How long have you had them? 1 male betta only. Had 2 years.
6. Were the fish placed under quarantine period? N/A
7. What temperature is the tank water currently? 76F
8. Are there live plants in the aquarium? No
9. What filter are you using? State brand, maintenance routine and power capacity. Whisper Power Filter 2-10 gal
10. Any other equipment used (aside from heater and filter which are two very important components of the tank)? No
11. Does your aquarium receive natural sunlight at any given part of the day? What is your lighting schedule (assuming you do not rely on sunlight for our viewing pleasure)? No direct sunlight. Many windows nearby
12. When did you perform your last water change and how much water was changed? How often do you change your water? Do you vacuum the substrate? Last change today, 25%, once/week, Yes.
13. What foods do you provide your fish? What is the feeding schedule? Hikari betta bio gold. A few pellets once/day
14. What unusual signs have you observed in your fish? Caudal Fin had a blister/bubble on the top edge and turned blue (normally purple). Blister popped. Fin started to decay at this site and decay is spreading in all directions: toward body and toward end of fin. The fins turn blue, then black and swollen, then fall off.
15. Have you treated your fish ahead of diagnosis? If so, what treatments did you use? State your reasons for planning ahead of proper diagnosis.
Yes, treatments of tetracycline and fungus cure. After much internet research and couldn't find anything, I figured it was either bacterial or fungal infection. After completing cycles of treatment as in directions, problem is still spreading.
16. Photo attached? Yes