There are a couple ways that top-off systems work. Some are strictly a float valve that is plumbed directly to your RO
, and some are a pump that has 2 sensors that detect the water level.
If your RO
has a auto shut-off solenoid on it, then a simple float valve is the cheapest and easiest. you just manually set the height of the float and when the water begins to drop below the set height, the float drops and opens the valve, and soon enough, you're back to your desired level. You won't ever see much fluctuation with this system because the float opens right away and keeps you constantly topped up.
The other one with a pump and sensors, is more expensive and more hassle in my mind, although it may be the only option if you can't plumb your RO
all the way to the tank. You would have a receptacle beside your tank with fresh water that would have a pump in it. When the sensor detect a lowered level in the sump, it would turn the pump on until the other sensor detected the appropriate level and shut the pump off.
This method still requires you to manually fill the receptacle, but it's your choice how large of one you have....you may be able to only fill it once a week.