LOL it is almost impossible. That is why folks rarely if ever recommend using a canister filter on a reef system. Canister filter + high lighting required for coral = algae.
Snails are not going to keep the glass spotless. No critter will. They eat in streaks, they don't care if they make it pretty
All tanks are going to have some amount of growth on the glass - I clean my glass usually 2 or 3 times a week. It is part of keeping an aquarium. However, at 20ppm nitrates, you are going to have more issues than most.