Not sure I totally understand your concept/design. Will this involve gravity feed or only ambient air pressure to provide the force to move the water? If it's the latter, I'm not sure that ambient pressure would suffice to keep your tank topped up consistently. Ambient air pressure tends to change with the weather.
I use something similar with the long/short rigid air line in a tightly stoppered glass wine carboy, but it uses an air pump on a timer to pressurize it. There's a check valve between the air pump and the carboy and a mechanical humidifier float valve in the display to regulate the level & prevent overfilling. The air check valve downstream of the air pump is to keep the carboy pressurized rather than to prevent water from feeding back since the shorter air line pretty much takes care of that. The mechanical float valve is mounted above the water level in the display, so feedback of saltwater back into the container is also a non issue. A future build will involve a basement sump and I plan to use the same principle, sans air pump/timer. The container will be mounted above the sump and gravity will take care of the flow from the top off container.
Low tech but reliable. It's been working very well since I got the timer schedule dialed in to keep up with the evaporation rate. Even if I run the timer longer than necessary, the small air pump is unlikely to overpressurize the carboy or pop the top off. The mechanical float switch does a great job of regulating the level since it's designed to deal with a home's water pressure which is substantially higher than what will ever come out of the carboy.
Possible failure scenarios include:
Timer/air pump failure = no top up resulting in level dropping;
Leak in any of the lines = same as timer/air pump failure and/or spillage on floor;
Blockage in any of the lines = same as timer/air pump failure;
Mechanical float switch stuck open = overfilling of display;
Mechanical float switch won't open/plugged = same as timer/air pump failure.
Most of these failure modes are unlikely with regular maintenance and observation.
Advantages aside from simplicity:
Minimum of electrics involved;
No electric float valves to worry about;
Top up rate is slow/consistent meaning water chemistry fluctuations are minimal & in event of float valve failure, overfill disaster takes some time to happen
Carboy holds about 7 gal US and is good for 5 to 6 days on my 77.
