What caused all my guppies to die, but all my other fish survive?

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.

Answer 1

you did not break in the tank proprely.

this is called new tank syndrome.

Answer 2

could be  the start of white spot do a 25% water change then add white spot/ich solution to the water it is made by inter-pet it is a 2 dose treatment day 1 then day 4 then on day 7 do a 30% water change to get rid of any infection/disease left in the water good luck