I ran a test on my 2x2x7 tray and pump last week to see how long it takes to flood.I have a 18 gal res and it only took 5 gals to flood the tray.But I put the 4-5" net pot with hydroton and the tray fill to the overflow with hydroton balls.I think the 18 gal will work all right I'am changing the res every 10 days and hope to only replace 3 or 4 gals between res change.20 gal res would be more stable and easyer.I have the same tray you do nt. I'll have some pixs this wkend,the cab is almost done.And the tests will start,heat ,water temps,the light tight test. Peace