Guys, I've been trying to set up a guppy tank for quite some time. After about 6 weeks of trying my guppies are still dying. Water quality so far seems good and I believe my tank is fully cycled. However the guppies keep getting fin rot and one just died. Another four seemed to have something going on with their fins and I am worried. Any suggestions on what I can do? Thanks!!

Btw, I got this dodgy medicine reccomended from the local aquatic store that says "Aquadine: gill fungus parasite special". I have quarantined the four ill looking guppies and added this medication. However I cannot find the active ingredient listed. I am wondering if I should add this medication into my main tank. I am worried that the medication will kill off the bacteria I have in my media.

Thanks again in advance for your kind advice.
May I know how you stopped the bacteria? Thanks!