They have had the setup for a year or so. Also they have a hang on filter that seems to be right size for the tank, way bigger than what I have at home. There is no overfeeding as well.How long has the tank been set up? It sounds like a bacterial bloom caused by the tank not being fully cycled. Only time can clear it up properly, though you can use a micron canister filter or a UV sterilizer to help the process along a bit.