Why do I have so much white fungus in my aquarium and how do I get rid of it?

I have had my 65 gallon set up forever and have had most of the fish in it for nearly a year…now they’re all dying, mainly the guppies. They are so hardy, why are they dying out rather than the more delicate breeds? I notice that when a fish comes up dead it’s already got at least a quarter inch of cottony white fungus on it. And I just noticed that a platy has a circle of it growing on it’s tail-AND ITS STILL ALIVE!! I admit I got lazy on the water changes for awhile, but I just did a 20-30% water change as usual and now suddenly they’re dropping like flies. What’s killing them? On a side note, I had some guppies who’s tails were chewed on pretty bad from a paradise fish I wrongly had in there, so I placed them in a breeder net in the same tank and added some Melafix(this scenario has worked fast and well in the past at healing there damaged tails), well this time 3 out of 5 died. Sorry for an unclear question, but what is killing the fish? The water quality? Or is it possible a high spore content is killing them somehow? It’s just crazy how fast they get fuzzy after death, so I know the water must be teaming with spores…I know what’s coming, no-I don’t know the specs on the pH and hardness and all that stuff, and I’m betting you will all say to do more pwc’s(which I am, don’t worry)…but mainly I want to know if the spore overgrowth could be the culprit and if so what is the best eradicator? Just the pwc’s or is there an evil chemical that is effective considering I have scaleless and delicate fish in there….OMG I’m a rambler, lol. Thanks for any help.