I use this top off system.
http://www.mops.ca/cgi-bin/SoftCart....html?E+scstore
and this pump......
http://www.mops.ca/cgi-bin/SoftCart....g+-p+DOOS-1137
My Ro unit fills a large , new, 120l garbage can. Which i control.
I have a small powerhead in the garbage can, just to keep the water moving. This system has not failed me yet, and has been running for a couple of years. It also is a source for my water change, water supply. For some reason I don't trust float valves, but that's just me.