drilling a hole

[louoner]

Ok so am going to drill a hole on a 20 gallon high. How many inches from the top should the hole be,thanks

 

[duke62]

is this for the drain or return.i usually make my return hole at least 2 1/2" from the top of glass.the drain i make lower for room for an elbow ansd a piece of pvc for adjustments for water level in tank.