What are you planning to keep in your tank? I do almost all SPS and have a crazy flow through my tank... probably somewhere in the range of 35x to 40x, but that is very turbulent flow so the actual water velocity isn't high enough that it lifts my sand. Again this is preference and I have found this through experimentation. For softies and LPS I'd go much less than this but some of my LPS. like my Ducans, love the flow.
As for flow through the sump, there are few different views on it. I'll give you the chemical engineering point of view first. If you treat the sump as a place to put your skimmer and are designing it as such, it actually doesn't matter what the flow rate is through the sump. Imagine your skimmer hanging off the back of your tank with no sump; it will process XXX GPH of that volume and, at least for this discussion, the skimmer doesn't really care a whole lot about how the water is moving around it. Now imagine grabbing a small section of your tank and stretching it off to one side so that the total volume is the same but part of it is just moved to one side; this is your sump. Put the skimmer there and look at the system again. Again, the skimmer processes XXX GPH of that total volume and it doesn't matter a whole lot how the water is moving around it. The biggest determining factor of skimming ability is the amount of contact time between the water and bubbles inside the skimmer, not necessarily how the water is moving around the skimmer.
Yes, if you match the flow through the sump to the flow through the skimmer, you can kind of, in very loose terms, say that all the water entering the sump is going through the skimmer. This in theory should give you a concentrated waste stream entering the skimmer and a purified stream leaving the skimmer. In a situation where you want to have a separate product stream of the most pure water you could get, this is what you want to do... but after we clean it, it gets sent right back to your display to mix up with all the other dirty water. So what is the point in this approach? (other than saving energy that is) In fact, a skimmer's efficiency goes down as the inlet concentration of waste goes up.
Put very simply, the skimmer is just an exit point for waste products from on otherwise closed system. The flow patterns around it don't, for the most part, matter. So, if you have a sump flow rate that far exceeds the skimmer's flow rate it doesn't matter; you just end up "cleaning" less water on a per pass basis but your total water volume will still be turned over the same (which will depend on your skimmer size) regardless of flow rate. So why not use the sump as a blend between a filtering section and closed loop system? I'm actually building a new system around this idea as we speak.
Now, from a practicality point of view, Myka is right. If you have a super high turn over through the sump you just end up pushing bubbles into the return pump and into your display. Bubble removal depends on the time it takes for the bubbles to reach the surface... so for a given flow rate, the bigger sump, the more time the water has to "de-bubble" before going back to the display. If you can remove the bubbles faster, then you can run a smaller sump. You and I are going to be running the same return flow, the only difference being our sump size. Mine is a 15G (not by choice but by space limitations) but I want a high flow to remove at least 1 or 2 powerheads from the display. Because of this I've had to come up with some fancy ways of removing bubbles faster to avoid the bubble issue with the faster flow.
There is also the matter of debris: a sump is a great place to settle out debris. So, just like the bubbles, a lower turn over will allow more debris to settle out before being sent back to the display.
Soooo... ranting finished... design your sump based on what you want it to do and your space requirements. Design what is right for you. Don't design it around the skimmer's flow rate. If you want it to have a high flow through it to act as a sort of closed loop, you can do it! You just need to keep practicality in mind while implementing it. Build the biggest sump you can to get the best performance but don't be afraid to be a bit more aggressive with your designs.
Also, I use the Mag 9 for the same pressure head as you. I'd use the Mag 9 over the 7 and throttle it back with a valve if needs be. I have no complaints with the Mag pumps but recommend Ehiem's if you can afford one of the higher flow pumps. Ehiem's are so nice and quiet.
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