I use an air pump on a timer to pressurize a 7 gallon glass wine carboy. This pushes water through a mechanical float valve to keep my display topped up for about a week. I like simple with a minimum of electrics if at all possible. Fewer failure points. If I had a sump, I'd elevate the top off container & use gravity (eliminating the air pump) to trickle water through the float valve. I've used this set up for a good 5 years now with no issues. Periodically dose alk, calcium, mag through the top off & never had the mechanical valve clog. Just need to ensure you mix whatever you're dosing real well & not overdo it. No a big deal to check the opening during a water change to ensure it's clear. I've shortened the valve by cutting off two of the float chambers to reduce the footprint in the display & it still works great.
Great bang for buck since I had most of the components kicking around the house already.
Solid rubber stopper drilled with two holes to accommodate rigid air tubes. Short one for air in, long one to the bottom to transport water to float valve.
Photo of the valve when it was still full length. I've shortened it by cutting off two sections at the end.
