I had the same problem when I ran my saltwater fish store. I sold a TON of water to customers. Someone setting a new tank up would borrow my 80 gallon plastic tank, toss it in the back of their truck and fill it with RO water. Not to mention the .5-1 gallon you sent home with every fish purchase.
The RO filter however was kind of wasteful. I got it down to about 1.5 gallons waste water to every gallon of filtered water it produced.
But I plumbed that usable waste water in to 4 300 gallon cubes in the basement and the dry cleaner plumped that water in to his washers and steamers so hardly any of it went down the drain with out being used.
My landlord showed up with the first water bill he got after we moved in. man was he livid.
I use my RO wastewater for most of the freshwater tanks. The plants and fish prefer hard water so this is perfect.