Recirc give the water more opportunity to be 'cleaned'. If the principal of a skimmer is to inject air in small bubbles unto the water thus causing the proteins or organic mater to collect on the bubble then when the bubble breaks the organic mater is collected in the cup.
With a recirc less water is moved through the skimmer but the water is forced to meet a lot more air bubbles and this is a more efficient when it comes to removing organic mater
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