Most of my tanks are drilled in the bottom ... so you use a stand pipe to regulate how high the water level is supposed to be ... as the water level rises, it flows into the stand pipe ...

However ... there are a lot of "homemade" drilled tanks that drill the sides of the tank (usually near the top) ... most of these use a 90 degree elbow attached to the bulkhead to regulate the water height ... I'm slowly working on a fish room and am considering this option because its cheaper to buy used aquariums and drill them myself ...

[A lot of new aquariums have tempered glass in the bottoms which you can not drill yourself] ...

Greg