Major problems in my Guppy/..

Recently all my guppies in my mix guppy/neon tank have been flashing off the substrate, 2 have already died, the first being a female who developed a large swelling and appeared to lose her skin, the second being a male whose tail became smaller and ragged at the edges, lost parts of his side fin and the became very lethargic, stayed near the top and did not chase females. I have no idea what the disease is, since my Neon Tetra's are not the least bit bothered, but I have more guppies whose tails are decreasing, becoming whitish around the edges, and everyone of them flashes from the oldest to the youngest fry. Any help on what the disease is, could be, and how to cure it would be greatly appreciated. I prefer alternatives (heat, salt etc, to chemicals but am prepared to use either or. It's not as bad as some pictures of fin-rot I've seen, and the tetras aren't effected so I doubt it's that, same for water parametres. Any help would be fantastic!

It sounds like the fish are having immune system prblems. Fish have fragile immune systems and over-stresswhen their water paramters are off, leading to disease. I would immeidatly change 50% of the water, go to the store and get FRESHWATER aquarium salt ( a good general cure for parasites and bacteria) and pick-up some water testing supplies too. If you can find out which of your water parameters (ammonia, Nitrate/nitrites, PH, etc.) are causing the issue, you can fix the problem medically.