I have a thin opaque scum that builds up about 24hrs after a water change that i can't seem to get rid of. I have tried adjusting the output from my external canister filter to be just under the surface and have good circulation and an air stone at night but it keeps returning. Co2 is good as confirmed by calibrated drop checker method, 55w AHS lighting over 16G tank & EI ferts. Glosso expanding, tonina, rotala occultifolia, blyxa, hair grass are all growing well. How do i get rid of it and stop it from returning