Ok thanksMy guess is if you have 2 filters, clean one and wait 2 weeks before cleaning the other one, that way if you mess up and kill the Benefitial Bacteria on 1, you still have the other filter to rely on .
With 2 filters if there is still areas where there isn’t surface agitation is that when I buy like air stones or do I know need ripples in the water every where on the surface?Breaking the surface is to help with gas exchange, if your filter is already causing movement you wouldn't need air bubblers to increase it. On a 40 i'd still do two as it never hurts to provide more filtration (also makes it easier because you can alternate filter cleanings so you do not lose your filter bacteria).
Waver makers are designed to cause no stagnant areas in the tank, or in the case of marine tanks where they are most often used to create current for there to be no place that has stagnant water or allowing live rock to stay live. If you have the filtration this is also an unnecessary thing in a freshwater tank - it is an underwater pump that moves water around more effectively because it can be positioned wherever in the tank - however I'd say a shark would try and swim into the propeller and possibly hurt itself (happens with the noodle shaped fish and often anemones) so you'd have to cover it with a stocking or similar to prevent any accidents.
I don't think you'd need a wave maker to get enough flow to prevent stagnant places in the tank, as well as it possibly being to strong for the long finned white skirts.