I've been trying to treat my betta fish for fin rot for a while now.

Fish normally lives in a planted, heated, and filtered 10 gallon aquarium. Only tank mates are trumpet snails. Tank parameters are generally \~50 nitrate ppm, 0 nitrite ppm, 0 ammonia ppm. Chlorine 0, PH 8, water is very hard. I've had the fish about 6 months, purchased from PetSmart. The tank has been established for over a year--I spent some time growing plants before adding the fish. 

Issue started with white areas on the ventral fin. Fish has been active and had a good appetite throughout. Betta is fed 4-5 Hikari Betta Bio Gold Pellets once daily. When increased cleaning of the primary tank didn't help, I moved the betta into a 3 gallon hospital tank. Tank has heater and  sponge filter. Plant is silk. When not treating with antibiotics, I've been doing 1/2 gallon water changes daily. When treating with antibiotics, I've been adding a few drops of Fritz Complete Water Treatment daily. Parameters have been as above, but with 0-10 nitrates ppm. 

So far I've tried: 

* 10 days of treatment with aquarium salt, going from 1 TBSP per gallon up to 3 TBSP per gallon. No improvement, and dorsal fin and ventral fin began degrading as well.
* Treated for 5 days with a combination of Maracyn and Ich-X. Rot stopped progressing during treatment, but resumed a couple days after completion.
* Treated for 10 days with a combination of Maracyn and Ich-X. Same result.
* Treated for 5 days with Maracyn 2. The caudal fin looks like it may actually have some regrowth, but the dorsal fin looks worse and the ventral fin is the same.

Does it look like the caudal fin is regrowing? Do I have the diagnosis wrong? What do I do now?

Fin Rot - Healing or Not?