You don't need much per day to sustain the water level. My 46+10g sump looses a little less than 1 gal per day. When you fill the tank, or do a water change, that's when you need the large volume. If you do a 10%w/c then it's like 5-6gal there.
I would say that just about any RO unit should work for you. It just might take a day or two to get enough water to fill the tank for the first time. _________ 1 true-percula & 2 fox-face firefish. 46-Bow w/ 20gal as sump. DIY wet/dry and DIY overflow. Coral Life skimmer. QuietOne return pump. Eheim ProII canister (just in case my DIY stuff fails). 36" Nova Extreme (2x39w 10k, 2x39w actinic with a fan loud enough to drowned out the noise from the tank) 48lbs. LR. Tank 2 is a 55G Fresh Water w/ 4 Discus. Tank 3 is a 10G fresh with Tetra. | 
