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When you say "diffuser", are you talking about the screen that goes over the pump intake? If so, then you want to use it for a skimmer that's in your main tank. If the skimmer is in the sump, I wouldn't bother with it.
I use the diffuser on the ps pump. Keeps small snails from getting sucked into that pump and into the skimmer. Now they can get over to the return area and I can return them to the main tank.
That's funny you ask. I have 2 excalibur skimmers (in-sump). One works better with it, and the other without. The pumps are rio 800. What kind of skimmer do you have and what pump?
it's built into my wet/dry, made by pro clear aquatics ( http://www.proclearaquatics.com/pro_wetdry.htm ), it uses a cap 2200 which i believe pushes 800gph. the pipe that connects the pump to the skimmer has an air flow valve built in so i don't think i need to use it with my pump.