Dare I ask how long you were gone that he dropped in 6 months worth of food?
Do massive water changes, back to back...50% or more if you are able to. While you are siphoning out the water, suck up the algae off the sand, and use a clean toothbrush to clean it from the rock and suck it up, as well.
If you don't have corals (or even if you do), lights out for 3-4 days. I would suggest minimal feeding (2-3 days a week)...the fish can live off their fat stores for a couple of days.