As i have explained above it is not a sickness that causes the above simptoms, it is the fact that your male guppys are stressing each other and the female species, males arnt to bright, and will also in some cases try to breed with themselves and other females, this causes stress, stress causes diseases. Add the two together and you = Death.
Suggestion: Reduce the male population in the tanks, make sure regular water changes are done, if you see a fish with clamped fins remove it to a tank by its own and feed it as much live and frozen foods as you can until it is healthy again.
If you see clamped fins DO NOT Medicate. in most cases it does more harm than good and can stress your fish out further, this is a common mistake made by many fish keepers, so dont worry to much.
Feed your fish on frozen and live foods regularly to help keep there diet in good order, which in turn increases there immune system, which can help against getting sick.
If as a result of stress, your fish contract a disease, try find a more natural silution to the illness, such as increaseing the silinity of the water to combat white spot and fin rot.
anything else i should have added people?
Shae