need help choosing a filtration device

[beach]

hello. i have a 55 gal reef tank at my office. i have a small sump and lots of rock. my protein skimmer is a hang on and probably 8 yrs old but skims ok. i do once a mo. 5 gal. ro/di water changes but i still have slime algae on rocks. a client suggested adding a filtration system like fluidized sand. any suggestions. thanks in advance. beach

[Timber77]

gfo or rowaphos will get rid of the phosphates that are feeding the slime algae. if your bulbs are old that can cause slime algae as well. maybe up the water changes a bit