Well my platy has given birth to a second batch of eight. Now I have 17 platies in the tank. I bought 2 algea eaters. One was larger than the other. The larger one is very active and has doublesd in size while the smaller one got more and more skinny until he looked awful. That night he died and now one of my danios has died as well. Both fish stayed in one spot on the floor for a day till they just died. Should this concern me with the other fish? The PH is 7. And I have very tiny Platy fry doing well for a week now.

Do you know what kind of algae eater you have? It's possible, if you didn't quarantine, that the new fish brought something into the tank such as a parasite. How big is the tank? Have you checked the ammonia and nitrite lately (the addition of new fry could have raised them up)? Hopefully the other fish won't be affected but keep an eye on them.