John's 75 Last picture

Overflow is top center and it is a drill through.

 

So isn't that still gravity fed? Could I do that but then just have a ball valve to regulate flow? I don't want to flood my sump and have water on the floor.

EDIT: Technically I guess one could say that all are gravity fed. What I am asking is Could I use a valve to regulate flow? What would you recommend?

 

The return pump regulates the flow through the sump. You build the sump so it has less volume used than it has capacity to handle any back siphon when there is a power outage. Make sure that there is enough room left to handle back flow. You drill 2 small anti-siphon holes in the return just below the level that the overflow start getting water into it.

 

I sort of understand what you mean. I have two boxes built in my sump(20g long) to handle overflow water. Each box is 12" tall, 6" x 3" wide(how many gallons is each?).

Here's my sump design. It's towards the middle of the 1st page.

http://www.3reef.com/forums/general-reef-topics/55g-build-indo-pacific-reef-88082.html

 

Each box is about .93 gallons. I think that will be a good sump.

If you look back at my sump you will see that the baffles are 12" tall. The tank is 15.5" tall to the cover lip. My sump is a 40 breeder so I have a space that is about 9 gallons for back flow.

You will need to have room for 2.5 to 5 gallons to handle back flow.

 

hmm. I have space for one more that same size. That would be a total of 2.79g. Or maybe I could do one large overflow area all the way across the back? that makes 3.7g

 

You could change the height of the baffles in the skimmer section to get more water room.

 

Well I was trying to have it so if the water reached over a certain level (12") water would go to the overflow area.

 

Please disregard this one post, I made a mistake and got confused...

These are my maximum dimensions I have room for in the sump. 2'5" x 12" x 3"

 

That is about 3.6 gallons. Make sure that the DT does not drop more than one inch when there is no power and you will be fine.