I've been searching all over in aquarium related forums for an answer to this questions and haven't been able to find a definitive answer. Background: Many saltwater reef aquariums utilize a filtration system located in a sub-tank (called a sump) beneath or remote of the display tank. Water drains from the display tank into the sump, goes through whatever filtration system is in place, and it pumped back into the display tank. The water level in the display tank is usually maintained by an "overflow box" which is simply a box inside the tank that is sealed in place, and there are holes drilled and bulkheads in place so that the water that flows over the top of this box then finds it's way into the drain plumbing. So if the power is lost, the water level in the tank only drops to the lowest point at the top of the overflow box. If you google "reef ready aquarium" you will see exactly what I am referring to. Most of these overflow towers have "teeth" along the top edge to prevent snails, fish, etc from escaping the tank and getting stuck in the plumbing system. I have been unable to find a reference of any type that definitively shows what flow rate can be expected through such teeth in various configurations. So, here is my question: What is the flow rate that can be achieved through a 1/4" wide slot? Being an engineer myself, I understand that at different water levels, there will be different flow rates. I am interested to know what the variance is as well. So let's clarify the question: I have a slot that is 1/4" wide and 2 inches tall. The bottom of the slot is square (generally they are rounded, but for simplicity, let's say they're squared off). The water level on one side of the slot is raised such that it is maintained at a constant level equal to 1/2" above the bottom of the slot. The water level on the other side of the slot is significantly lower (this is not relevant, because as long as the water level is not equal to or higher than the bottom of the slot, the water will exit the slot at the same rate). What is the flow rate through this slot? What is the flow rate if the water level is raised to 3/4"? 1"? 1.25"? etc. I'm sure there is an equation that covers this, that's what I'm looking for. I figure that there has to be some component of it that takes into account viscosity and turbulence or something. For instance, if the slot was 1/8" wide the flow rate would likely be less than half of the 1/4", and conversely, if the slot was 1/2" wide the flow rate would likely be greater than doubled. So if anyone can help me out or at least point me in the right direction, it would be much appreciated!