so i added a small sump to my 20g mixed reef. The sump is just an acrylic box that fits behind the tank. I opted to not go with an overflow box. THe siphon tube was just to likley to loose siphon especially if the power went out. I made an overflow out of pvc and it works great. I used 3/4 in pvc it would give me the gph i needed. I faked multiple two hour power outages and the overflow never lost its siphon here are pictures:
This is the overflow:
Water goes in through the strainer at the top. then up over the side of the tank and out into the sump. The open ended pipe is an air release and the primer. I put a check valve in the top to suck out any air bubbles that get caught that way I have a perfect siphon, or close to it
Here it is in the tank:
