I just finished a DIY ATO on my 72 gal with 25 gal sump , it cost me about $30 & an hour & works great . I use bottled RO water for top up so I just drilled a bottle top , inserted 2 - 1/4" clear vinyl tubes.
1 is connected to an Aqua lifter pump that sends the water to a 2 way valve on the sump then 1 side of the valve is connected to a $6 humidifier float from Home Depot , the other side of the valve acts as a recirc line back to the water bottle when the float shuts off. When the bottle is empty all it takes is a cap swap & its good for another 3-4 days.
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