My Platies Are Dying

[Iiriva]

Moving to another tank is always stressfull to a fish, no matter what kind it is. And quick changes in the type of water could cause all kind of problems as well.

Sometimes pH can change in your own tank as well, if you add a new root, or if the tap water changes... And usually causes problems it it happens too fast. There are many options, but I'm sure that when you get your water results back, we are closer to the right one.

 

[snazy]

I think people confuse sometimes wild fish with domestically bred fish.
How is a platy that was born in soft water will ever have any issue with it?
If you bought it from a shop or someone that kept the fish in harder water, then maybe..But still the ones you grow yourself in your own soft water should have no issues with lifetime and sickness.

@ MarcC

Maybe your new fish killed the platies, or bullied them to death? I think platies get stressed very easily.
It's still a good idea to get the test kit and check your water parameters, but it's strange that the platies died first if that's the issue.
Another possibility that I always consider when I see no external signs is parasites of some kind, but still why kill the platies so fast and not the newcomers.
Smells like fish incompatibility at this stage, but I am only speculating.

 

[snazy]

Just to add to the above that also there is no need to worry about the Ph that much altogether, as the minerals in the water that platies "may" need to last 4 years as someone said may be increased easily with either salt or just a simple all purpose plant fertilizer.