Why did all of my guppies die yet most of other fish are fine?

Question

The tank was about a month old and they started to die off one by one, swimming on their sides and backs and eventually breathing rate slowed down and they died. Initially i thought it was ammonia poisoning, changed the water and treated the water with finrot medicine and internal bacterial infection medicine but to no avail. However, if it was ammonnia why did it only effect the guppies?

What are some reasons for guppy deaths? I thought they were supposed to be hardy?

They shared the tank with non agressive mollies, 2 gouramies, platies and neons.

Also one platie died after a white blob appeared in place of a missing fin, and now the other platie is getting a whiet mark. One gouramie died after becoming very lethargic... now the other orange gouramie is losing colour and becoming whiter...please help any advice to any of my questions if you can. thanks.

((BUt all the other fish are fine and have been all their lives. I want to be able to add guppies again someday, how long to wait etc what to do?))

Answer 1

The guppies may have just died naturally- they are a hardy fish, but only live for 6 months tops. Or at least, thats what the Petsmart employee said. :)

Answer 2

How big is your tank? You need at least a 30 and up gallon tank. That blob on your platy was probably a tummor. And your guppies had swiming bladder disease. Did you cycle your tank and test it? Cycling a tank is a good way to get rid of ammonia and nitrites and lower the pH.to get rid of swiming bladder disease all you do is peel a cooked green pea mash it feed it to your fish. For the tumor you have to go to the vet imediatly!

Also if the blob was white its fungus. To cure that I would use a medicine like pimafix by API

Edit

Tallking to a petsmart worker is like talking to a wall they don't know anything