Variation on the gravity feed that's worked flawlessly for me for better than two years. No sump, impractical for me to elevate top off container above display, so I do need to cheat a little to help gravity along.
Uses air pump on a timer, 7 gal glass wine carboy pressurized by air pump, some rigid & flexible tubing, stopper with two holes, mechanical house humidifier float valve on a DIY acrylic bracket. Takes a bit of tuning to get the timing for the air pump so that it keeps the car boy sufficiently pressurized as it slowly empties, but once done, no further adjustment required. In my case, the water level in the display needs to have a close tolerance otherwise the overflow box housing the pump feeding my HOB skimmer will cavitate. This system has been rock solid in keeping water level constant with a slow trickle. Folks with a sump will be able to get away with a bit more variation with regard to water level without any disastrous consequences.
Occasionally I'll throw some calcium, epsom salts or baked baking soda in the top off if I find any of the pertinent parameters are getting a little too far out of the ballpark. I will use gravity fed once I get a new setup with basement sump. The fewer components you use, the less can go wrong. You can throw all the money you wish at backups & failsafes, but every component you add will also add the possibility of another thing that can go wrong. I'll take a guess and say that I may have spent 50 bucks on all the components. Several I already had kicking around & I don't have to go back to a specific manufacturer to replace any one of them.
I've posted these photos of my ATO setup a number of times now, some of you are probably getting sick up & fed of seeing them, but for the benefit of those that haven't....
Glass wine carboy, 7 gal, give or take. Sits on a stand next to display & keeps it topped up for 7 days. Short tube is air in, long one water out to mechanical float valve. The timer kicks in for a few minutes every hour to keep the carboy pressurized. Even if there's a power failure, there will still be residual air pressure to continue topping off for some time if required.
Mechanical float valve. Not a sealed unit, but open chambers at bottom that trap air to lift it, so will not ever get water logged. It has a wheel on the top that allows for additional water level adjustment. I've recently shortened it by cutting one chamber off the end for a smaller in tank footprint. Still works like a charm. Very slow trickle into the display due to relatively low air pressure provided by the small air pump. You'd really have to be out to lunch for a few days for this to screw up & either over fill or not fill. It has never clogged even though I've mixed kalk, alk & mag into the carboy top off water.
