i had mr albertas old cube.
i had the closed loop in take in the overflow(coast to coast).mag 24
and a durso intake for the sump.mag 18 return
when the power went out water only went down to the durso level.
the closed loop had no effect on the level in the sump.

typically a closed loop is used to get flow into the tank without power heads, if you used a large pump on your closed loop there simply isn't enough water going through the teeth on the overflow box to keep the pump functioning, you might be getting away with this with a medium sized pump, but to recommend this to the average reefer is asking for trouble.

Dursos will control the height of the overflow box but returns from the sump into the tank will also cause back flow, as can be seen from Des pic his system will fail, as will any other
system that isnt don exactly correctly, why ask for trouble.

not only that, the correct way to use a feed to a closed loop pump is the get the goodness out of the rock and into the water column, you use the pump feed to enhance movement in the tank, doing this from the overflow box means you are losing the flow you could have used to your advantage, your teeth in the overflow box need to be low enough for the CL to get enough water, if this is the case then you are not surface skimming which is the whole point of the overflow box.

Ya OK, my system will NEVER work......................

I didn't say your system wouldn't work, your closed loop pump is 700 gph, the teeth can handle that quantity, you cannot do this with a 3000gph pump it simply will drain overflow box.

Well, I would think that it would be apparant that the overflow is going to have to be large enough to handle whatever flow pump you run. There was never the question, "will my Pump be to big for this overflow".

Unfortunately Dan nothing is always apparent in this hobby, hence my reason for suggesting against doing this.

sorry should have added.
closed loop mag 24
return was a mag 18.

saltynuts, that's a fair amount of flow with those 2 pumps. What is the weir length of the overflow that is handling that flow?

I had a mag 18 and it flowed ALOT. to much for my drains to handle, thats why I went the 2 smaller pumps

overflow was a coast to coast
1 icnh for closed loop mag 24
2 inch for sump durso. mag 18

Ahh, CtC I missed that. My next tank, I would like to do a CtC