It took me 3 days to fill my 125g, it's perfectly normal. You don't have to do it all at once. Plus once you put the rock and sand in you won't need 75g of water due to water displacement. You may end up with 60g or less of actual water. My 29g biocube I believe holds exactly 18g of water.
From my research everywhere it appears that the 75gpd units are the norm and produce the most amount of water the quickest with the least amount of waste water. The plus unit has a gauge on it for water pressure too, I was going to see if I have the 65psi needed to add a booster membrane that would up the gpd to 150 and result in a 1:1 ratio as far as pure water and wast water.
This is the addon membrane:
150 GPD Water Saving Upgrade Kit - English