my advice is dont buy it until you need it. If the fish has something that will kill it in a day then meds will probably just add stress and hury the untimely death up faster.
Heat is the best cure for most things except bacterial, Melafix is good for that.
Any time my betta got sick, I put the temp to 30C for a few days and problem solved. Fin rot is caused by bad water quality so sort that fist and foremost or no cure will work effectively long term.
For persistent parasite problems, Quick Cure is a good all round remedy and your best bet as most diseases are too hard to pinpoint anyway.
Water changes, Water changes, Water changes.
Good Luck
Ken