I've had my school of 12 pygmy Cory's for about 3 weeks now, but lately one has been sitting in the corner of the tank or at the bottom not moving but he will occasionally join the shoal and act normal as if nothing was wrong. All the others seem very active and healthy. My water chemistry all check out normal and temp is just right. they're in a heavily planted 20 Gallon long with 4 platys and a gourami. I have a sand bottom first layer (petco brand) and a fluorite bottom layer.