how many times a hour should the water go through the sump?

[golf nut]

Originally Posted by MikeInToronto

Now if your argument is that a low-flow overflow extracts surfactants better, perhaps you can discuss that more.

That is the whole point Mike.

Firstly we all have overflow commonly called prefilter boxes...Why call them this?

Because they are moving the surface water only from the tank when done properly, If you imagined that fish waste, coral waste and detritus was similar to oil it isn't hard to visualize oil floating on the surface of the DT, the prefilter box is there to remove it, the slower the removal the more concentrated that would be , if the extraction rate is increased the oil is mixed with more water and becomes diluted.
So this oil/ water mix then goes to the sump where some use filter sock for containing large bits etc, it then should enter the skimmer as concentrated as possible and then the skimmer can do it's job properly.

Why anyone would want to keep returning dirty water continuously back to the tank so they can keep skimming a little at a time is beyond me, get it out get rid of it and return clean water back to the tank.