Betta Fish: Any advice on setting up a hospital tank?

So my betta lapis has quite bad fin rot, and I think I should set up a hospital tank. I recently upgraded her tank so I still have three old one which is about 18L, her new one is 21L. I'm using wunder tonic, as that's what I've got, and I've tried 2 aquarium salt treatments which have done nothing so I figure it's time to pull out the big guns. Even tho I've been keeping fish for about a year and a half, I'm new to treating them for illness, so I've done some reading on hospital tanks but I wanna hear advice specific to bettas. Should I move lapis to her old tank, or can i get away with using her current tank to house her (all it has is a plastic Hutt and some gravel) and once she's better do sometime like a 90% water change to get rid of the meds? Do I need to cycle the hospital tank? What temp should it be at? Should I use the filter? (her current tank is a sponge filter and her old tank is like a sponge combined with those filter media rings) Also if anyone has used wunder tonic before and knows if it's good or not.