I'm wondering if there is any way to calculate the amount of water that will flow back into sump when pump is turned off?

[ozs]

I'm in the process of linking a 60 gallon cube with a 120. The cube is not drilled yet. My sump is only 20 gallons but I really don't want to replace it.

 

[mann1139]

You can do a rough calculation if you know the square footage of your tanks (length vs depth) and the distance the water will drop if the pump turns off.

Display Tank (l x d x drop) / Sump (l x d) = Rise in your sump.

 

[mfinn]

I'm in the process of linking a 60 gallon cube with a 120. The cube is not drilled yet. My sump is only 20 gallons but I really don't want to replace it.

You can try calculating it ahead of time, but nothing beats a freshwater test. Fill with freshwater, turn pumps on and then turn them off. Start with enough water to just cover the return pump. Mark the spot where it settles out and go from there.
By using loc line on the inside of the tank on the return line you do have a little wiggle room for adjustments.

 

[ozs]

Thanks for the help. I'm gonna post a build thread once I get into it.

 

[Duane family]

Its real easy, take the length and width of the inside of the tank and multiply that by 10. This will give you a value for how much water will flow per 10mm of height. Then measure the height from full water line till where the flow will stop and you can have your answer.

As an example:
1200mm length by 500mm width will be 6Ltrs per 10mm of water drop.
If the flow will stop after 50mm of water drop then the total amount of water removed from the display will be
6Ltr x 5 = 30Ltr