What kind of water are you using?
What is your PO4 testing at?
Using a canister filter, and if you are not using RODI water, it will be difficult to maintain a suitable nitrate level, not only to house coral, but also to prevent the growth of nuisance algae. 20ppm nitrate in a reef tank is on the high side, and algae will be able to easily capitalize on this excess of nutrients in the system.