through the sump is fine if you construct your baffles in a way that will control the bubbles. There will be a lot of microbubbles with that much water dumping into the drain end of the sump. What do you want in the sump as far as equipment? Will you be using an in-sump skimmer? If you wanted to do a mud sump, that flow would be about right. With a mud/caulerpa sump, you have a fuge incorporated into your sump. Refugiums typically are separate tanks with relatively low flow...which is fine if that's what you want. Do a Google on Ecosystem mud filters and read up on the theory behind that type of sump and see if it intrigues you at all. I can tell you from experience that it works. If you have the money to spend, it's nice to order one of their sumps and just plug it all in and start it up. If you're like me and paying all the monthly bills is a juggling act, then know that you can build your own. How much room do you have under the stand? If I remember correctly (which is always suspect), a 40 breeder is 36x18x13. If you go with a 50 tall, you have the same dimensions except that it's 17" tall instead of 13". Now you can run 30 gal
instead of 20 and still have 10 gal
of "extra" room. Ok...now IMO
, you don't need that. Your main tank will always drain down the same amount...every time. So, you get the tank running with some water in the sump. Shut it all off and let it drain down as much as it's going to. Fill the sump on up till it's about 1" from the top. Turn the pumps back on and mark the new operating level of the sump. This is where you can safely fill it to. In a tank with a 36"x18" footprint, you have about 2.8 gal
per inch. By closing the margin, you gained about 7gal of water in the system. I don't mean to make this sound more complicated that it really is, but a poorly designed sump is a pain...a well designed one makes tank maintenance and upkeep much easier. Tell me what you want to run in the sump and I'll try to be more specific. If you're just going to use the sump to house equipment, then I'd probably be inclined to drop the flow to about 1000gph and run a closed loop pump in the 1200-1500gph range for a total flow of about 2200-2500gph or 20x tank size. JMHO.