For auto top off with the ro/di, I would definitely not hook it up directly to a float in the sump. I set mine up with triple redundancy to avoid flooding the tank which happened once before I had all the safety measures in place :/. The current setup is about a 10gallon reservoir tank, I used a nice air tight dog food bin that is tall and skinny from wal-mart. I have the reservoir hooked up directly to the ro/di but I purchased an electronic solenoid valve which is normal closed position and opens when power is applied in between the ro/di and the reservoir. I hooked up the solenoid to a timer which only allows it to turn on for 30min each morning to refill, being in the garage my 250g total system volume evaporates about 5gallons per day. Then in the sump I have a float switch hooked up to the feed pump in the reservoir and a 2nd float switch an inch above it hooked up in opposite way, so when water drops in the sump even one centimeter, the feed pump is powered on and tops off the tank and shuts off when the float reaches the top of its sensor limit, and if it gets stuck on(damn snails), as soon as the 2nd sensor above the first senses a lift (it floats) it kills power to the pump. Lots of redundancy in this setup and I've yet to have a flood with this setup and I love it.