Size of sump tank?

[Jessica]

I need a bit of help...i have a grow bed that is 92.5" x 42.25" x 15"...1/2 to 3/4" gravel will be used. I am guessing that the sump tank will be approximately 215.52 gallons? Can someone please help me with this issue, before I go nuts???

[George]

There are probably volume calculators on the internet.  Your gravel bed water volume will be approx 40%.  For example, a 100 gallon gravel bed holds approximately 40 gallons of water.  If you have or intend to have one such gravel bed, then a 50 gallon sump would be sufficient.

[Jeff S]

George is correct but I'd go with a little larger sump, maybe 75 gallons so you don't have to keep close tabs on water loss.