04-18-2011, 01:38 PM
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I can't really speak to surface slime...except to say that nearly all stagnant pools seem to have it!
A canister filter alone does not oxygenate the water unless the return line is above the water line in some way. Now I'm not necessarily suggesting a spray bar. I have a couple of HOB power filters in play and I also have a bubble wall on left side of the tank (as you can see in my avitar). I believe that bubbles breaking the surface not only highly oxygenate the water but also aid in the release some unwanted gases. If your tank is heavily planted with living plants, perhaps this is less of an issue. If/when fish are gulping at the surface, I think I'd suspect O2 depletion in the water if I didn't have ample mechanical aeration.