Check out filterdirect or Aquasafe Canada. Both have good units. Most of the 6 stage units are really only 5 stage. The 6th stage is a carbon filter for taste. Not needed for the fish but good for drinking.
I would think you could fill the tank and then put a separate valve in before the 6th stage to fill a container for the tank when needed.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,