How much is too much?

[NewReefer455]

I started my tank out as a 15g cube and a 20gal sump. I elected to use a ehiem 1250 return pump (320gph at zero head) and ran it with 1/2" flex up to the tank. I eventually added a short 5 gal frag tank and decided to run another ehiem 1250 for a return because two of them fit perfectly in my pump chamber. I have just recently upgraded to a 40 breeder from the 25 cube and frag tank, but I am using the same sump setup with dual pumps. Because I already had them, I decided to run both eheims to the tank so if a pump ever failed there would still be circulation and heated water getting to the display. It was unintentional, but the backup is definitely a plus and it also helps to keep flow in the tank when doing my 6-month return pump cleanings.

When I first got the tank running it was cobbled together with the plumbing from the original tanks just so that I could quickly get all livestock and sand transferred over to avoid any mishaps or mini-cycles. Everything went smoothly and I have been running the tank for 3 or 4 weeks now in the currently configuration without any issues. Just yesterday I began to finally fix up the plumbing and decided to bump up to 3/4" return line to get a little more flow out of the pump. I'm using coralvue 3/4" bulkheads so that I can bump up a pipe size without modifying anything. I pulled some water out and swapped a single bulkhead to start and I put a plug in it so that I could start plumbing the one return while the other could still get water flowing from the sump. I just now swapped returns and I am running the new return plumbing while I plumb out the second one, and I was not expecting such a change in flow. I'll need to get out an old fluid dynamics textbook to get the actual numbers, but it seems like this single 3/4" return is putting out more flow than both the pumps did previously with the 1/2" plumbing combined.

Should I be worried about increasing the turnover through the sump this significantly, or was I just undersized to begin with? The majority of the sump is a fuge and I have been growing red gracilaria like a weed. I have a mandarin, scooter blenny, and two wrasses and this sump has been more than sufficient at supplying all of them with an influx of pods, I just don't want to be putting too much flow through the sump to the point that I may decrease the number of pods in the system. Am I worrying for nothing?

 

[jassermd]

It's going to increase the flow exponentially... Poiseuille's Law - F = πPr^4 / 8nl

To put this relationship into words: At a given temperature, flow rate through a tube or pipe is inversely proportional to the length of the tube the viscosity of the liquid. Flow rate is directly proportional to the pressure gradient and the fourth power of the radius of the pipe.
Even when turbulence is a factor, you can still use Poiseuille's equation to get a reasonably accurate idea of the how flow rate changes with pipe diameter. Keep in mind that the stated size of a pipe is a measure of its diameter, and you need the radius to apply Poiseuille's law.
Suppose you have a length of 2-inch water pipe, and you want to know how much the flow rate will increase if you replace it with 6-inch pipe. That's a change in radius of 2 inches. Assume the length of the pipe and the pressure are constant. The temperature of the water should also be constant, because the viscosity of water increases as the temperature decreases. If all these conditions are met, the flow rate will change by a factor of 2^4, or 16.
Not sure about the turnover in the sump; it's going to go up given that you increased the flow exponentially. You can always tune your return pumps down to decrease the amount of flow based on the new plumbing.