That sump does not hold much water, I am suppressed it over flowed the tank even if the level was high. Here is what I think. If the level of the water in the tank has to be so high that the volume of that small sump will over flow lots of water onto the floor, then the water level in that tank is just too high. Some overflow boxes are adjustable in height. Try lowering it all the way so the goose neck is resting on the edge of the tank. If this can not be done for some reason then I would recommend a different over flow box to allow you to do so on that tank. Second you do not want the amount of water in the sump to be overflowed if the power goes out or return pump stops for some reason. That may be hard to do because of the small size (percentage of the tank) you have in the sump. You may have to experiment a bit with water level and turning on and off everything to see where the levels will rest if something goes wrong. I have a feeling you will have a very small volume of working area between overflowing, working correctly, and sucking air into the return pump. You may even have to put marks on the sump and keep the water levels in the sweet spot when running in a daily basis. You do not have to run the sump at is maximum GPH, you can install a valve on the outlet of the pump. 100-200gph would be fine for its purposes IMO. You can use a closed loop and or power heads in the tank for more water flow in the tank.