I just noticed that you started out with the Tetracycline. You still want to get some of the other meds indicated but since the fish is in such dire straits, I'd start with what you have on hand until you can get something more effective.
Tetracycline isn't as effective in hard water and is said to work best on gram positive bacteria so that might need to be taken into consideration. It should not harm your plants and if your betta is the only tank inhabitant, you can treat in the 10 gallon.