Corals are out of the water for hours at a time when the tide goes out but if you plan everything ahead of time then they probably won't be out of the water more then half an hour. I drained and pumped back in over 160g of water in an hour and a half.
The only thing you might want to do if you have sand in the tank is to make a little well at the front of the tank for the fish to go into so they don't hide at the back. I managed to catch 5 fish that way without moving a single rock
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