How much return flow?

This question depends on alot of things and who you talk to. lol. You will want water turnover in the tank to keep the water oxygenated. Now, that can be done by moving alot of water through the sump and also by powerheads circulating water in the tank. I have heard that slower flow through the sump is good for the refugium part (if you have one) as well as the skimming of the water. So, if you have a few powerheads that can move some good water, then you should be able to keep the water oxygenated as well as flowing for corals well enough with lower flow through the sump.

To the question about the ball valve on the pump. You don't want to be throttling down the pump. This will cause unnecessary problems to the pump down the road by over working it. If you want to slow the flow of the pump, I would consider a ball valve on a T'd in release. So, you have the pump going up to the tank with no ball. Instead, you put in a T connection and that T connection runs back to your sump right by your pump. You then put a ball valve on that return to the pump. To throttle down the pump, you just slightly open the ball valve on that connection. What happens then is you divert some of the water back to the sump so you have less flow going into the display tank. This gives you some throttle on your pump without straining your pump.

Hope this helps and that others with expertise can chime in to help.

 

It depends on how much you are throttling back. A little bit of flow restriction is ok. It is similar to adding more pressure head. However, if you throttle down say close to 50% you can cause backpressure in the pump as well as strain that can heat up the pump and shorten the pumps life.

 

This is great idea and the way i have mine set up. I use that flow back to return bump as the flow to go thru my reactor and it works great.

 

i agree the best way is to use a T and ball valve and flow back to sump or other equipment or add more tankls like i also did

 

also on my other tank i have 58 gallon with a sump that i really only flow like 200-300 gallons throught it an hour but have two korlina 750's in the tank and on the other tanks my return pump is a mag 12 and that sump flows a lot of water so its pretty much up to u both ways have worked fine for me so far

 

　I know this may sound wrong but it actually works the opposite of this. The more water you move or pump, the more energy it takes to move it. Throttling back a pumps output does not harm it in anyway and on the upside it actually causes the pump to use LESS electricity and produce LESS heat. You can check this yourself by using a kill-watt-meter as you turn back the flow. On the other side teeing off your return back to sump does no harm eaither it does waste some electricity though.
As far as to how much flow through the sump there is not exact number since there is a lot of variables involved. Some run 10 times their aquarium volume some run less.  As long as your able to maintain water quality it's enough. I run a 350 SPS reef and my actual return flow is only around 400gph. I run a eheim 1262 return pump which is rated at 900gph @ 0 head and pulls 80 watts. After head loss, running through a chiller and 120 watt UV it drops down to about 400gph and is now only pulling 35 watts of electricity!
The key to flow in a reef is random turbulent flow you don't want lots constant laminar flow. This is where wave makers come in.