In my old set up i had about 25’ run from where my RODI was and the sump of my tank. It took me a while to figure out the best way to get the top offs done. I had the RODI water filtered into a holding tank then had it pumped to the tank. then one day when the system failed and RODI water pumped all over the carpet…. and while the wife was yelling at me….. I figured it out..
I ran water line from the RODI system UP the wall and across the ceiling Then down the wall across the room and down to the sump level then though the wall to the sump. Mounted the RODI system near the Washer’s water line. Installed a Y splitter to the washer line (one to washer and one to a Battery operated Garden Watering timer, I connected the RODI to the Garden watering timer. I placed a splitter on the RODI lines going to the tank as well as a shut off valve. Ran another water line to a holding tank (where i mixed my saltwater) and installed a shut off. On each water line (mixing tank and tank sump) I installed a FLOAT VALVE and adjusted the float to shut the water off at the correct “top off level”. With the display tank fully topped off i marked the sump where the correct level was with a marker, visual markers are always a good thing.
Once that was all done, I waited until the tank needed to be ”topped off” then went to turn on the valve at the garden water timer. I figured out the amount of time the system needed to be running to correctly top off my tank, I think it was about 5-8 minutes Or something like that. I tested this out for a few days to be sure. Once i was sure, i set the garden timer to OPEN the valve at 6pm for 10 minutes (remember the float valve will shut off the water flow once it reached the correct levels i set earlier). Whenever i needed to mix saltwater I would just shut off the valve to the tank and open the other (figured out how long it took to make 20 gallons and added 5 minutes, again the float valve shuts the flow off once filled to the desired level)
After i had this system fine tuned. I went 3 years with no floods and no issues with fish or corals before i took the tank down (we moved) and gave my wife the opportunity to complain about other things around the house.
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all together it cost me a BATTERY OPERATED Garden hose timer, two float valve kits, a roll of water line, a Y adapter and some zip ties. Maybe this could work for you.