adding to already set up tank

[camarojones]

hi so i have a 60 gallon tank an i plan to eventually change me tank size to 120 but for now i just have the 60g so i have 1 tiger oscar cichlid, 1black convict cichlid, 1jack dempsy cichlid, 8 electric yellow cichlid, 4 violet gobbys, 1 chinese alge eater, 1 pelocostomus, 2 cory catfish 1opal gourami, 2 crawfish. i want to add more bottom feeders to this tank i dont know what to add i also want to change the gravel on the bottom to half sand.. my tank is at a point of homeostasis i would say because the fish have been living to get now for a 6 months except for the 8 electric yellow the were just added a month ago... any thought on what to add an how to manage