[center:9c1b67a9b7] Welcome to AA, pamelam!!
Definately try algae wafers.
There are a few other things that may be problematic in the tank. What are you water parameters (ammonia, nitrite, nitrate)? How long has the tank been going? When did you add fish? How large is the tank? How many/what types on fish are present in the tank? Finally, how often do you vacuum the gravel and do water changes?
Did all the platies die at the same time? Were they bloated looking like they over ate? Were there any signs of illness?