Hi there
I have three tanks an 850 litre a 400 and a 200 the 400 is set up with a sump as you describe and is superb, i am going to convert my 850 to the same system.
Our set up as follows if of any interest.
Overflow from main tank enters the first chamber which has course wool on the top then noodles bio bals and shells water coming out the bottom into the second large chamber, the water here is always good quality we just have a pump in this part of the sump that pumps back to the main tank, we have a valve on the otlet side of the pump to regulate flow back to the main tank.
A trap to watch, if you are setting this up yourself, is to make sure the outlet backinto the main tank is only just below the max level of your main tank, as when the power goes off the main tank will siphen back through the inlet untli it equalises, so you need a bit of spare capacity there as well.
The other advantage is you can put your heater etc in the sump, so nothing in the main tank.
Hope this helps.
Cheers