for the past 20 years, I have used a simple 'air line' valve to constantly drip RO water into my sump. Everyone that has seen my system, has seen this, in fact, I was using the same holding container to top off, a 130g SPS tank, and my main system which was about 1000g. I adjust it more or less based on the evaporation rates. It has never failed, the trick is that my fresh RO water goes into a small holding container, controlled by a simple float valve (which has and will fail), but when there is a problem the container has an overflow drilled into it, so any excess water goes down the drain, NOT into the tank! The few times my skimmer has gone 'crazy' and wasted 4-10 gallons (or more) of salt water, I certainly didn't want an ATO replacing it with fresh water. The current system, which is 7 years old, cost less than $100, and is still good to go!
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