If you bought a RO
unit with a solenoid on it (auto shut-off) then you can plumb the output line to a rubbermaid tub of desired size and install a float valve ($20) that will shut your RO
off after the tub is full (you can buy an add-on solenoid if your's doesn't have one). Then all you need to do is dip a jug into it for top-offs and when you need to do water changes, you can dump it into another tub for mixing (or use a small powerhead) .
Ideally, if you can swing it, route another line from the RO
(split with a T fitting) to the tanks' sump and put a float valve in that. then you don't need to worry about top-offs, they're done for you automatically and keep your system at a constant water level.