I recently bought Otto cats to clean my brown algae, well it took them two days and they ate it all. I bought algae waffers to supplement their diet but they never go for them and my blood fin tetras eat it all anyways. Is there anything else I can do to feed them? I remember seeing once in someones tank a sponge on the bottom of their tank and algae eaters sucking on it, does anyone know what that was?