Thoughts on Overflow

[Mathew0]

In my research I have found that most large tanks (over 150 gallons) have an over flow on both the left and right sides of the tank.

What are your thoughts on overflows for large tanks?
How many should they have?
What flow rate do you look for?
Do you wish your tank had a 2nd, or didn't?
Do you think the number of return lines determines the number of overflows?

 

[blaxsun]

Or an overflow in the center. My thoughts are regardless of placement, you want a minimum of two overflow pipes (main, emergency) with each overflow. I aim for 3-5x flow myself for the main return. I'm actually fine with the one (center) overflow in my tank, as all the overflow/return plumbing runs in one area. Not necessarily - the overflow pipe(s) simply need to exceed the capacity of any return lines. More returns simply enable more distributed flow in the tank.

 

[Mathew0]

Does anyone else have ideas on overflows?
GPH vs. number of times turnover?
Perhaps you use your return as a wavemaker and base your overflow volume on that?

 

[All_talk]

I think 3x to 5x the tank volume is a good place to start if you have good in-tank flow from wavemakers. Back in the day we tried to use return flow to move water in the tank but that never work all that well. I've heard that its a good idea to have a overflow/return flow that is equal or greater than you skimmers flow. I can see the logic in that but I don't know that its real critical. Personally I would target 5X tank volume but be sure that my overflow/plumbing and return could do a good bit more.

As to number and placement of overflows, I don't think that its critical, let your logistical requirements and aesthetic preferences dictate. But I do see some logic in having the return(s) somewhat opposite of the overflow. I would also add that redundant return pumps make good sense to me, and those work best with separate returns for each pump.

 

[exnisstech]

On my 6ft 180g I have two HOB overflows, one on each end with the return in the center split into two return nozzles. I have no idea how much turn over I'm getting. I have a reef flow hammerhead with a gate valve to control flow. I adjust the return flow to the max that the overflows can handle and depend on powerheads for the in tank flow but I have no idea how much that is. The amount of flow is not something I actual have ever tried to calculate.
If I ever get around to having a custom tank built I would like to do a coast to coast external.

 

[Pntbll687]

Does anyone else have ideas on overflows?
GPH vs. number of times turnover?
Perhaps you use your return as a wavemaker and base your overflow volume on that?

GPH vs turnover is going to depend on the pipe size and type of drains you have. Most "mass produced" tanks Aqueon/Marineland will have two holes in each overflow. The aqueon ones are drilled to fit two 1" bulkheads. If you go herbie drain style (one full siphon drain and one emergency) then you'll get just about 1000gph through a 1in pipe at full siphon. Depending on the tank size that could be anywhere from 5x to 20x turnover.

Using the random flow or pulsing on a return pump will cause issues with the drains, losing siphon, or too much flow and causing a gurgle noise from the emergency drain. It will also cause the sump level to fluctuate, and if you have an ATO it will go off constantly.

If you go with a single drain (durso style) which comes with most tanks, you can still get about 700gph through them without them getting too noisy.

My 180g peninsula from Aqueon has a single overflow. I run a full siphon drain with 3/4" pvc and a durso pipe as well.