Eh, the water hardness probably wasn't anything to do with the fin problem, hardwater fish kept in soft water suffer problems from the lack of minerals, it's more a long term problem that affects them internally, and shortens their lifespan. You wouldn't see visual evidence after just a few weeks in soft water. You're far from the only person who hasn't known about the importance of GH either, I kept otos (soft water fish) in my hard water for nine months, oblivious, since they seemed to be thriving, until I came here and learned about water hardness and the effects in has. I now have them in softer water, but I did lose three with no other obvious cause over the next few months. For soft water fish, the excess minerals in hard water build up internally, eventually causing internal damage like blocking their kidneys. There isn't anything you can see externally, they just pass away much sooner than they should.
For worms, you need the right meds, nothing else will get rid of them. But you have the right meds now
Hmm, I'm suspecting that this isn't 'fin rot'. I reckon the danios are shredding the guppies fins. I tried some guppies in a tank with some schools of tetra and zebra danios for a short while, and the guppies tails were shredded, just like the ones in your last posts. I don't know for sure whether it was the neons or danios, but I once I moved the guppies into a different tank, no danios or tetra, their tails healed up just fine. Had no issues with guppies in any of my other tanks, only in that one. It was other fish shredding them, not the water.
Sorry, it's hard to keep track since you have a bunch of different threads, but what's your complete stocking? What size tanks, with how many of what fish? Want to try to help you fix this, and it's better to rule out the simple things before throwing different meds at them for fin rot after treating the worms, only to discover later that it's not bacterial or water quality, but danios tearing their fins, or the male guppies ripping each others tails. Especially since you've been trying to battle this for six months and nothing has worked before... makes it even more likely that it's the danios or something else in the tank with them. Worms don't cause fin rot or torn shredded fins.