"Charles Spitzer" wrote in message ...
i'm curious, why would increasing the size of the sump work? i would think
that since the pump is emptying it, eventually either the pump will empty
the sump and run dry, or the tank will overflow because not enough water is
coming out of it.

It is quite simple. The flow of water through the overflow
depends on the difference of water levels between the tank
and the overflow other side...
The bigger the difference the greater the flow.

At the begining, when you turn your pump on, the pump
pumps normaly fast but the overflow does not let the water
go fast enough... Over time, when the water in the tank
rises, the pressure build up on both sides of the overflow
syphon and the speed of water flowing through the overflow
increases until it will equalize with the pump output...

And now, if it takes more than 3 gallons to make up
the big enough water level difference on both sides
of the overflow then the sump will run dry before the
overflow will reach its maximum water output...

I would propose an experiment... :-)
Take a big bucket, like an empty Instant Ocean salt container.
Its volume is about 6 gallons. (the new style is smaller...)
Drop your submersible pump into the bucket, low on the
bottom and fill up the bucket with the salt mix (same
temperature, salnity to be mixed with your tank water).
Drop the hose from the overflow to the bucket as well...
Turn on the pump and see that your overflow is good enough :-)

All you need is a bigger sump. At least bigger pump
compartment. Your tank is simply too big for the sump
and the water in the tank does not rise fast enough
to fuel the overflow.