lingering bacteria problem

I had a bacterial infection problem recently with my Gold Pristella Tetras that I treated with clear liquid Anti-Bacteria stuff, I think made by Jungle purchased at Petsmart. Anyway, there were no instructions on the bottle except for the dosage amount. Since the sick fish didn't seem to improve after a couple doses (ulcers, imbalance, red gills) I was ultimately putting in about a half-dose everyday. After about 2 weeks, one healthy looking GPT died and then 4 days later the really sick looking one finally died (1 remains). About 2 weeks after that, one of my Red Serpae tetras died (now 3 remain). NOW the remaining GPT is having balance issues (sinking tail, etc), I assume due to bacteria effecting his swim bladder, and he still has red gills, which I've read are a sign of bacteria infection.

Also, feeling desperate, I've been adding salt to the tank lately, since I've read in places that it's kind of a "fix-all". Any truth to this?

Since my first round of treatments with what I think was a bacterial infection did not work too well, any ideas what to do with the remaining GPT and in the future? Or was I just way off thinking it was bacteria?!

details

20 gal
3 black mollies
1 gold pristella tetra (2 recently died)
3 red serpae tetra (1 recently died)
1 Oto
2 white clouds

I vaccum the tank and do about a 30-40% water change once a month. I try not to stir up the gravel too much as to not get an algae bloom (had that problem once before). I also change the filter media and put in new carbon. I just read that may not be necessary and changing the media removes a lot of the good bacteria. It just looks so nasty and smells, I used to think it was a source of my probelems. But every month, the last one I put in is just as nasty as the one it replaced.

Mardel 5in1 strips say Ammonia/Nitrite are 0 (doesn't really have "Ammonia" reading), Nitrate in the 30-40ppm range. I know I should try to reduce that, but when I feed them less, the Nitrate doesn't seem to go down.

Red gills - the gill plates became pink/reddish. Nothing sticking out.
Ulcers - were ulcers. Not sure what else I could say.
Imbalance - the fish's tail sinks - he makes quick moves to stay upright. This has carried on for about 3 weeks now. His tail is also slightly bent downward, so I assume the illness has created some sort of dysplasia - again leading me to believe it's a bacteria problem. He hasn't been eating and I notice this morning that his "gut" is now getting smaller.

The only symptom I could see on the red serpae was that his lip appeared white for many weeks, but it never became a cottony, fungus-looking growth and never spread or worsened. He was just bright red and one day when I got home...dead. Never had funny behavior.