06-05-2010, 11:38 AM
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As I have this issue I'll add my comments. The "scum" is protein, absolutely natural in any aquarium, though with surface movement it is usually not present. Different biology in different tanks means that some tanks have it, others never seem to. It is not harmful unless it thickens to the point of preventing the gas exchange at the surface; in a planted tank this is less of a concern. A slight surface movement is fine, it is the "overkill" that has to be avoided which is why I frequently say "no surface disturbance." There is a vast difference between the "disturbance" from an Aqua-Clear waterfall type filter or a powerhead, and that created by your Rena if positioned correctly.
I use the spraybar on my 70g and 90g tanks, but on the 115g which happens to have a Rena XP3 [good filters I think] I also removed the spraybar but I did this because the 5-foot hose will not reach from the filter to the far end of the tank, and going to their site they do not have extended hoses. So I have the spigot as far along the back as it will go, and it is directed at the far end wall, about a foot beyond the spigot. The water flow is sufficient to create a very minimal surface movement at that end, but not too much.