I can keep spot alive in here and even shad for several hours as long as I don't put too many in there. I think it will hold a good 3 - 5 dozen finger mullet or maybe a couple dozen spot or shad.
I'd estimate it to be about 12 gallons or maybe a little more? It does have bubblers on each side. If its hot out, I will change the water and/or throw ice in there occasionally.
