1) Remove the Aquarium salt. (Do a water change using water that does NOT contain aquarium salt.)
Aquarium salt is for external issues only. If she doesn't have wounds or fin rot, then it is not needed.
Aquarium salt is also high in sodium. This can cause bloating, and puts stress on the kidneys as they try to eliminate the excess fluid.
2) Add Epsom salt.
Epsom salt has a laxative/fluid reducing effect. I recommend using OFL's Epsom salt treatment. It starts out with a dosage of approx 1 teaspoon Epsom salt per gal on the first day, then brings it up to 3 teaspoons per gallon over the course of several days. The entire treatment can be done for 10-14 days.
OFL's Epsom salt treatment:
- Add 3 teaspoons Epsom salt to one gal of conditioned water.
- Stir/shake until the Epsom salt is completely dissolved.
- Using this premixed treatment water, make 25% water changes every 15 min for 1 hour today.
- Tomorrow, do a 50% water change using this premixed treatment water.
- Then, every other day for 10-14 days, do a 50% water change using this premixed treatment water.
Epsom salt is inexpensive, and you can get it at supermarkets and grocery stores, such as Walmart, CVS, Walgreens, Publix, etc.
Look for 100% magnesium sulfate. (No dyes, perfumes or additives.) Since it doesn't contain sodium, it won't stress his kidneys (like aquarium salt does). Look for 100% magnesium sulfate. (No dyes, additives or perfumes.)
You can also cover the top of the tank with plastic vegetable wrap. This will increase humidity levels, and make it easier to breathe. Be sure to leave several inches of air between the water and wrap, so your fish can breathe!
3) Since other fish have died - you should treat the tank.
I would use a broad spectrum antibiotic, such as the sulfa class. API Triple Sulfa
or Mardel Maracyn Plus
(must be "plus"). These are available at places like Petsmart. Do not use this if you are allergic to sulfa drugs.
Another antibiotic option is Kanaplex
(Kanamycin). This isn't available at many petstores, so call ahead before you go out. If you can't get it locally, it can be ordered online.
Antibiotics will kill the bacteria in your filter too, so remove the media before treating. (But use caution when replacing it later, since it may be harboring whatever pathogen is killing your fish.)
4) Do NOT put more than one Betta into a single tank (unless the tank is divided).
They will fight. One or both will be injured, or possibly killed.
5) Do NOT put a sick betta into a healthy betta's tank.
You don't know what's wrong with her. Putting her into the male's tank will expose him to the same pathogens that are affecting her.
Do not share any equipment between tanks. You do not want to cross-contaminate the other tank.