Cleaning overflow help

[Tsev03]

i have a cpr overflow box and it's so nasty in places I can't get to. How do I clean it? How long can my tank go without the overflow box while I try to clean it?
Tank is 125 mixed reef. I have the power heads at the top of the water level sucking some air into the tank

 

[dbl]

Take it off and give it a diluted vinegar bath. Just have the powerhead breaking the surface while it's off. You can do all that within an hour and you'll be fine, even if it takes longer than that. Give it a good rinse and put it back in service.