PlumRT70, Hello! I have a 240 gallon, and used to use a 40 or so gallon container as both a RO DI storage tank for future saltwater mix's and as a top off source for evaporation purposes. It worked well, but not a 100 percent! The tank was off grade, enclosed in its own room, so spills weren't a problem, but you could easily have 40 gallons in your tank due to a malfunction along the line some where. I now use a 7 gallon bucket, a float switch(pressure), and a Magdrive 500. When the levels in the sump change to the eqivalent of about 3 or 4 gallons, the pressure switch kicks the mag on for just a few seconds, maybe once a day. Salinity is right on, and the risk in a failure is 7 gallons, plus the RO DI refill. A lot better than 40 or more gallons.
I have a seperate 32 gallon container for salt water mix now, that is tee'd into the RO DI output. I have the RO DI on an automatic shut off valve so that the unit shuts off when there is no load on the unit. So far so good! Steve