Unfortuantely with RO
units, the waste to "good" water is like 80-20.
Me, when I had a larger saltwater, I cheated on the first fill-up. I used distilled water for half, then filled up the other half with spring water. It was pricey but it was better than wasting about 100 gallons of water
and I was able to fill the tank immedaitely instead of it taking 3 days.
I also cheated in that I didn't have liverock or sand in the tank when I filled it. I just filled the tank with water and then dumped in the salt according to level I needed and the tank gallonage. With two powerheads, the salt mixed nicely in about 30 hours and then I proceeded to add the sand and liverock. Once I got the initial cycle done, I then started using RO
for my water changes.
That tank ran beautifully for three years until I had to move and ended up converting to freshwater due to a job change and desire for a lower maintenance tank.