Oh ok, than your water changing schedule is good. When I first came on this website, I didn't know much about diseases or what medications to use and I put Betafix (watered down version of Melafix) in a tank that had sick fish and so many people told me to take them out of the water ASAP because they had problems with it also. It can burn their labrinth muscle so I freaked and took them out immediately.
Pet stores will sell anything and usually pet store employees don't know what they're talking about, so I never trust them. When I first started researching bettas online a website said that you should be ready for anything and make sure you have medication handy because your fish will likely get sick. I thought to myself, yeah right, mine will be fine.lol I should've listened to that person because I would've saved myself a lot of time and money if I had everything there. So one day I had free time and looked up all the different diseases, the simptoms of each of them etc so I know what to look for. Google is a VERY good thing. :P