Fish Rescue! But he's sicker than I realized

Hi Everyone,

(I posted here a few months ago after was able to rescue a little guy from a petland with bad fin rot, and just want to say it's been 6 months and he's doing great! His tail has grown back and his coloring is beautiful, so thanks to all that helped me with advice!)

I am going to need some advice again!
I went back yesterday but of course was distracted by all of the sick babies, and decided to point it out and ask to take the two sickest. (Does anyone else have the experience of the clerk being repulsed at how sick the fish are, when it's like...YOU work here, YOU could have prevented this?!)

Anyway, I have two little guys here--one seems to be doing okay, but the other seems to be much sicker than I originally noticed. I'll have to try to add a photo, but I'm having a hard time photographing what I can see.

It looks like he may be a delta tail but it is so hard to tell, his caudal fin is completely gone. I'm not sure how fast body rot takes to set in, but there is no trace of caudal fin. There is whiteish tissue drifting off of his dorsal and anal fin, he has white cottony patches on his body and anal fin. I didn't notice it until I had him in a jar (moving him) that magnified him a lot.
He has been laying at the bottom of the tank for the past 12 hours or so, breathing heavy (could that be gill damage?). I was "slow and steady" in moving them into the new environments in the hopes to not stress them out any further!!

I set up a 10g tank (it's only partially full...i filled it to the same level the 2g would be--the other betta is in the 2g), water parameters ok, 80 degrees, and I only used aquarium salt & water conditioner. Only some fake plants and a little bit of gravel and a terra cotta pot. My last guy I had in a 2g where I did 100% water changes every day to every other day, and only added aquarium salt (and water conditioner) and he made a fast recovery. I was thinking of going out and getting another 2g tank and doing the same thing, but considering how sick he looks I'm not sure if that would be too stressful, to do the 100% changes everyday?

And I am confused about medications--I've read in so many places to always be stocked up on betta medications, but then I've read in other places using medicines is just more damaging and clean water and aquarium salt will work just as well. At the moment I have Bettafix and Melafix but have not used either. Considering that is all I did with my other guy, I was confident that the water changes and aqaurium salt would work well again but now I am having second thoughts.


Sorry this is getting very long.
Any questions/suggestions appreciated!! Thanks for your help in advance!