my central system works on gravity. the pump feeds the top tanks, and the overflows fall into tanks below it, until water is finally returned to the sump. unless you keep these tanks bare glass, and/or compliment each tank with a sponge filter, you run the risk of constant ick breakouts (aka LFS syndrome). the trick is to keep your water moving. 15 tanks, averaging 20 gal each, will require a system capable of managing a 1200-1500 gph flow rate. IME.