Hi everyone. Have had my tank set up since Xmas. Fish since mid February. Everything was ok till last week. Now platys are staying at the bottom of the tank. One of my yellow/black platys has gone missing and another is in the baby tank as it was resting on the bottom of the tank. He has laboured breathing and is not moving I have looked everywhere for missing fish. I have done 3 half water changes. I recently added some biological media to filter. Do u think this is the problem? I also have an algae eater. He does sometimes chase the fish could he have injured him/her. I also have baby fish they seem fine and are swimming all around the tank And the harliguins are also fine.