I usually do a 30% change every week, including gravel vacuuming. Those plecos sure can poo! At the time I had my cories, though, they were in a 10 gallon, which I had a REALLY hard time maintaining. Things are dying a lot less iwth my 20. I still have the 10, and would like to use it now that I'm moving and have some more room, but I'm not sure what I should put in it. I'm thinking a couple of moors, I've always liked them, just for show.