Is that a
betta or a guppy? I saw someone say betta but I think it's a guppy so I'll go with that and mention a few different things...
-I find guppies thrive much more in harder, less acidic water, and from what I've seen they will often really struggle at a
pH below 7.
Crushed coral or
wonder shells are both really great ways to raise pH and hardness, but if you need more, I would recommend getting a product like
seachem equilibrium of
cichlid salt and mixing it in when you do water changes.
-You're probably already on it, but keep working on getting that
ammonia reading down to zero, and make sure your
nitrite is at zero as well, and your
nitrate is under 40 ppm. Is this a new tank that's still getting established, or an ammonia spike in an older tank?
-Guppies can handle a lot of salt for treatment. What else is in your tank? If there are only other guppies and no
live plants, I would add 1 tablespoon of
aquarium salt per 2 gallons of water and maintain that
salinity for at least a week, and if there's no improvement, make it 1 tablespoon per gallon. If you have live plants or other fish that might be more sensitive to salt, do you have a
quarantine tank you can use? I'll talk more about this if you need.
-If you're in the states, it might not be a bad idea to get some antibiotics in there to help you fight that, since it's a pretty bad case and it might help to get some big guns in there. I find good water will help fight off mild or new infections, but this might be too much for that. Erythromycin would be my recommendation.