I'm looking into upgrading my freshwater 90g to a 135 or larger and will go to a sump for the 1st time. I will build the sump from a glass tank and have an inlet chamber, 1-2 media chambers, + the pump chamber with heater. I'm not getting how to determine how many gph will flow thru my sump. I got to thinking today when looking at 2 external overflow boxes (CPR?) (my preference is to do internal). The first one had one 1 1/2" bulkhead rated for 1600gph and the other had 2 1" bulkheads for a total of 1500gph. Going way back to high school math I found that the 1 1/2" outlet had more surface area, therefor more flow, that 2 1" outlets.

If I plan on having 2 outlets, either 1" or 1 1/2", how do I figure out the flow rate into my sump? It has something to do with the depth of the water in the aquarium, or the distance from the top of the durso standpipe to the sump chamber? The higher the fall, the faster the fall, the higher the gph flow rate? How is this calculated? How do I create my desired turnover rate? I guess I'm not getting how you can manipulate flow rate when the outlet size remains constant...how do you "slow down" gravity?

I've read that the strength of the pump will determine flow rate (I understand pump ratings vary based on how high you are pushing the water back up to the tank)? I don't get this...isn't gravity gravity and water will run downhill at a constant rate when the aquarium level stays constant?

I can't get how the sump doesn't overflow if the pump doesn't at the very least push out as much water as flows in via the aquarium outlet. Unless it is because the reserved volume of the sump is greater than the volume of water in the plumbing and the volume of water from the aquarium surface down to the overflow box level. This is the safety net in the event the pump quits (ie power outage).

And why are there 2 returns of 1" when the pump's outlet is 1". Why not just have 1 1" return? If you start with 1" outlet, how does splitting that into 2 1" outlets at some arbitrary point affect anything? I'm not clear why it seems to be suggested to have 2 returns entering the tank rather than just 1 (unless it is just to have 2 re-entry points to create more water/tank disturbance).

For a guy that excelled in the sciences this has got the best of me....anyone make this clear as mud for me? OK, hope this post doesn't leave you now completely confused with my questions!!! :-?

Thanks.