Not that I would try this without extensive testing...Theoretically could you not skip the float valve and use simple physics for a gravity fed overflow?
like those self filling dog/cat water bowls..
thinking about it more as I type it probably wouldn't work...a bottle or holder with a pipe into the return section with a small hole just below where you need the water level. as water evaporates and the hole hits the surface FW would flow and when the hole was submerged again it should stop.
I have no idea honestly just reading this thread reminded me of those dog bowls.
Hate to be the one to resurrect an ancient thread, but it still appears to be one of the top threads in the DIY section for me so maybe this will still help someone else.
Another reef keeper used the exact idea you mentioned, and has proven it works well (for nano aquariums, at least):
https://www.nano-reef.com/forums/topic/172031-nano-sapiens-12g-ye-olde-mixed-reef/#comments
The first page of the thread has comments detailing exactly how it works and an interesting experiment, but as a quick summary:
They took a hamster water bottle, removed the metal ball inside the “straw”, and fixed it upside down above the tank full of water. As the tank water evaporates, the water bottle drains into the tank to maintain the same water level.
On a small aquarium, one water bottle can last a couple days; the person in the linked thread managed to sustain the water level of a 12g aquarium for a week using this and a makeshift lid, iirc.
Seems to be dirt cheap and very little effort for fairly good results, depending on tank size.
