I currently have a Red Eyed Tetra, A Male and female Platy, (the only survivors)

Everything else we seem to buy drops dead, a few of our female guppies died, and shortly after we noticed little baby guppies in the tank, which seem to be thriving.

What could be causing any new additions to die? We have already used some Water Treatment that our local aquariam recomended as a just in case.

We are getting tierd of buying fish just for them to drop like wildfire and die.
I will Get the water tested as soon as i can!!!
Our tank is planted with live plants that also seem to be absolutley thriving (and growing pretty well)

Any Advice would be greatly appreciated