So, this is the simplest auto-topoff probably ever. It's just gravity fed with a float valve.
Firstly I installed a shelf above my sump.
Then I got this float valve from foster and smith:
Then I attatched it to the return compartment of my sump. Had to turn it sideways and mount it on the front because of space issues but it still works the same.
Then I attached the RO
tubing to my water reservoir (4.25 gallon plastic container) using a "quick connect male adapter". I actually got the hole so perfect I was able to thread the fitting through the plastic container.. so the fit is TIGHT. Then I siliconed around the inside and outside. I am going to order a nylon nut to secure it though.
(sideways picture again)
Now I'm just waiting for the silicone to cure before I leak test it. Finally! Stable salinity without me worrying about daily topoffs and my pump running half dry. Now I only need to worry about filling up the reservoir.
Here's my whole laboratory underneath the tank