Snail,red algie

I have a 130 gal salt tank year and half old. First 6 months, after setup time of two month, went great. Filtering with canister filter Rena 3 and hang on protein skimmer. Also have a circulation pump on one end. Tank temp is 77 degrees. Problems I am having red algie, and red slime, snail keep dieing. Fish, blue legged crabs and banded shrimp are fine. I do monthly filter cleans and skimmer doesn't pick up as much as it used to. Clean live sand every three months. Any suggestions