90g Reef. Need Help Setting Up.

So I decided against drilling the tank. I bought an overflow box today and I'm going to use my 30g as a sump. I found a spot to cut the acrylic up top to where the tank can be used with an overflow box.

In the picture I put on this post, you can see the three holes. I'm going to cut about 3/4in back and wide enough to hold the box. I talked to a couple different people before deciding. The only thing with the overflow box I'm worried about is what happens if the power goes out? Is there any way to stop the suction or will water just start poring out?

 

Actually, the overflow box is not the issue; the return piping is. The overflow box will determine the height of the water in the tank. Once power goes out, water will still drain like normal until the water level lowers below the teeth of the box. This is a very minimal amount of water. However, if you have your return piping low in the tank, it will siphon back into the tank until it is lower than the water level as well. So a few things you can do are to keep the return(s) high in the tank, use a check valve to minimize how much water will flow back, and drill a small hole in your return piping just below the water level. Once the water lowers below this hole it will suck air and break the siphon.

 

Thank you, that actually really will help. I didn't think about it like that so I appreciate the advice.