in my opiona 20 long isnt very deep for a sump.the taller the better.you want as big as a sump as you can fit under your main tank.you want room for your filter sock,your return pump,your refugium,your heater,and your protein skimmer.if you use the 20 long by the time you make your step downs you will only have a couple of gallons of water in the return side and will have to add water more often.i would say at least a 29 gallon if you cant go bigger.i have made several they are easy and a lot cheaper then buying one.