Thread title contains 'gravity' but you mention 'air' in your first post. True gravity feed would have the top up reservoir above the vessel you are topping off. Head pressure would then feed the liquid through a simple mechanical valve into the tank or sump.
If you're talking about pressurizing the top off reservoir with air which will push the water through the tubing, the process is similar, except for the extra tubing for air into the reservoir. That is the set up I've been using successfully for many years. 7 gallon glass carboy, timer, air pump, tubing, mechanical float valve. The mechanical valve is one made for furnace humidifier trays available for about $10 at most hardware retailers. My timer runs a few minutes every hour to keep the carboy under pressure. I modified a rubber stopper with 2 holes, one for air in & one for water out to the tank. Rigid air lines go through the stopper & are connected to flexible tubing. I used scrap acrylic to fashion a bracket that holds the mechanical valve in position on the rim of my display. Nylon screws hold the bracket securely. I drilled & tapped holes in the bracket for the nylon screws.
Glass carboy keeps my 77 gallon topped up for about 7 days.
'Wait" brand mechanical valve. I cut two chambers off this later to shorten it making less of a footprint in the display. Still works fine.
