Those are pretty hardy too. I wouldn't keep the water warmer than 80F for the fish. Average room temp. should be fine.

Do you keep them in with the snapper?

Rosies require a lot of oxygen in the water. You need a decent filter with surface disturbance(the kind that hang on the back of the tank) and a bubbler wouldn't hurt.

Here's a Rosy care site...
http://www.fishpondinfo.com/rosies.htm

You might want to try a few different pet stores. You may have better luck if you can find healthier fish to start with.

Hope this helps. Good luck.