Probably a lot but I wouldn't get rid of the stone. Plants only produce oxygen during light. At night they remove oxygen from the water and by the morning the level will be low. Although they will produce more oxygen during the day than they use at night so giving a nett production if oxygen you will have a fluctuating oxygen level which may stress the fish.
I'm using a Fluval 207 canister filter with a lily pipe outflow that will sit at the surface of the water. My tank is 36"x12"x12" , I should probably use a wave pump to create current also because of the length.