Nothing.
I only have a pair of ocellaris, a pair of mandarins and a few PJ cardinals in my 125g.
I have two little 'coves' in my rock work. One was claimed by the clownfish the other one is where I put the grazing station. It's nice to finally have 6 feet of tank instead of just 3'!
If they wanted to come in it would be okay too. My main concern was to be able to leave the food in there and be able to get it back out after a few hours.
In tanks with bigger competition the size of the hole would keep some fish out