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.
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