The trouble with the idea is that the R/O filter will remove water volume from the tank, for every 2 litres of water filtered at least 1 litre is waste water that goes down a drain. If you are running say a 50 gallon per day unit for 4 hours a day then you are going to lose 3 or 4 gallons of water from the tank every time you run it.
The other problem is going to be achieving enough water pressure to get the R/O filter to work, the membranes need the water forced onto them at mains water pressure and in some areas they even need a booster pump set up on the incoming main to get the pressure up high enough, so to run one on a tank you will need a very large powerfull pump.
How about setting one up on a constant drip & drain system ? If you ran an R/O unit off the mains 24 hours a day and had a small hole drilled in the tank to act as an overflow to take excess volume away to a drain so the tank doesnt over fill you would effectively have a constant water change going on, you would just have to add a small ammount of trace elements to the tank every couple of days, although i'm sure that i've seen a 7 day auto dosing pump somewhere, Eheim prehaps?
