As you know, evaporated water in your reef aquarium or fish-only saltwater aquarium needs to be substituted with fresh water since salt does not evaporate.
I have witnessed many people use electric float valves directly plugged into their sumps which turn on a water pump when the water drops below a certain point. In my opinion, this is a disaster waiting to happen unless of course there are several safeties built-in, this can be expensive and still not 100% fail-proof.
If you still insist on using an electrical float and pump system, I recommend looking no further than the Neptune ATK V2 auto top-off. I have some buddies who use it and they are pleased. If you want to buy Neptune ATK V2 auto top-off for your aquarium then you can visit https://www.marinedepot.com/neptune-atk-v2-auto-top-off-kit.
If you want a Do-it-yourself gravity auto top-off, please continue reading.
I have been using a gravity feed along with a float set-up for the past 4 years without having any issues. If kept clean (clean algae and causation of the sump float valve) it will in no way overfill your sump with freshwater unless some freak accident happens.
Here is a checklist of what you will need to build your own auto top-off.
An RO unit (for both top-ups and water changes) a small water reservoir tank, an auto shut off pressure valve, a ¼" 1-way valve, an optional ¼" solenoid to switch main water to RO on and off, an electronic timer to control the solenoid, two plastic float valves, some ¼ tube, a 1/4″ bulkhead to hook up the ¼" tube from the reservoir to the sump valve.