That.. sounds a lot like ich to me. Here's a fish with typical ich white spots. Did it look like this?
If so, your fish has ich and he has it because his water is dirty and slightly too cold. The spot didn't vanish, it dropped off due to the salt and hatched - if the salt was not correct dosage or you stopped using it too early, it released loads of wiggly parasites into the water, infesting his gills.
Once this bout is gone, do more weekly water changes. Bring the heat up to 78-80. Hopefully, no more ich.
Stop using bettafix.. Get him into some anti-protozoan medication containing malachite green - ask for one that's not harsh (I used Protozin, wonderful stuff). There's also the salt/heat method but if the ich is infesting his gills so badly, I'd put him on meds pretty quick.