Overflow Box question.

[N1Husker]

What is the purpose of an overflow box such as the CPR HOB overflow box as opposed to just a tube in the aquarium connected to a submersible pump in a sump?

 

[user 26995]

An overflow box will match the flow out of the tank the same as the flow from the return pump back into the tank as long as the return is not too strong for it. Check the specs on the overflow box for the GPH it can handle.
The overflow box will not break the siphon if you were to turn the return pump off and it will not allow your tank to empty into and overflow your sump. So it will start back up when you turn the return pump back on.
If all you have is a hose from the tank to the sump, it will keep running water to the sump until it sucks air and breaks the siphon. Risking overflowing your sump. Then you would have to start the siphon again when you turn the return pump back on or it will empty your sump into and overflow your tank.

 

[N1Husker]

So if the overflow box says that it is rated at 300 gph, then what gph rating should I get for the submersible pumps to bring water into the sump and then to return it to the aquarium. Also I read that I should drill two small holes in the return flow tube in the aquarium just below the water line to break suction to prevent overflow. What is your opinion?

 

[user 26995]

Definately drill the holes. I drilled two 1/8" holes in my overflow tube just above the tank waterline.

Check the specs on the submersible pumps they will tell you what the gph is at specific heights.
Eheim pumps are my prefered return pump.
Water Currents and Movement in Aquariums: Eheim Hobby Pumps