Hi there
i recently set up my a 26 gallon tank that i had for tropical fish
i did the fishless cycle and once everything had cycled i added
2 dwarf gouramis
1 betta
6 platys

the dwarf gouramis and the betta are doing great

i got one platy from a lfs and 5 others from pet supplies plus
the ones from pet supplies plus were a little ratty looking, two fish had some tears in their fins
the day i got the platys one of the platys was dead
the one that died was the biggest and seemed pretty healthy
i just took that one as a freak thing

a few days later another platy, Perdita, was dead
this one didn't seem to be doing as well in the days before death
she had a clamped dorsal fin alot and seemed to just kinda be off on her own alot and didn't play around with the other guys. her fins were also ratty looking but she came like that

the next day sunny, and gordita were dead
sunny was the one from the lfs
he seemed to be doing great he was one of the most active ones
Gordita did have clamped fins but otherwise seemed fine

1 of my remaing platys, Chalupa, doesn't seem to be doing good
Chalupa has clamped fins and has two irresdiesnt white spots
one spot also goes on two his dorsal fin
it doesn't look like fungus because the patches are flat
fins fins also are frayed but when i got him it was like that

the other platy Clemintine seems to be fine

doesn't anyone know what is killing my platys? ammonia and nitrite are both fine
I really need help before the last two bite the dust

today Bonnie the dwarf gourami died.
Chalupa isn't doing good he is laying at the bottom of the tank not really moving

http://www.fishlore.com/Disease.htm

Here's a link to a website that has some fish diseases listed and what to do for it. Sounds like your platies might have some kind of fungus--white patches and ratty fins could be fungus. Platies like a little aquarium salt so that might be all you need but check out the link for other meds, too. Hope it helps!

Good Luck with your sick fishies!

The rate at which they died, and the fact that they're livebearers and are particularly prone to it, it sounds like you've got a columnaris infection. White fuzzy patches near the mouth and on the fins right? I'd treat for columnaris. This disease spreads so fast, it's probably not the fish shop's fault, it frequently comes in from the distributors. -When I worked in an aquarium we used to have four or five tanks dedicated to guppies, just because when a new batch arrived from the distributor we didn't want to mix the new with the old in case they had columnaris, we used to find out in about 24-48hours when half the new batch would just die there and then. and if we'd put them in with healthy guppies the healthy ones would die within about 48-72 hours. maddening.

You did do waterchanges to get the params down before adding them, right? Sorry if that's a stupid question, but other than that they must have been sick before you got them.