Top scum?

Can you get a picture? Scum could be anything from algae to cynobacteria to something that fell in the tank and got stuck there.

Is it a sort of oily cloudy residue on the surface? If so it's just typical surface scum which I believe is just an accumulation of organic waste and such. I think it can come from dead plant matter, fish waste, or even just fish food. Increased surface flow should fix it.

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I remove my surface scum by laying paper towel on it then removing the towel before it sinks. Works great for me.

I agree that it is likely a protein scum, caused by organics. I have none at the moment in any of my six running tanks, but I have in the past seen it in this or that tank. During the water change, invert the hose toward the surface and it is fairly easy to deal with. It will of course return, depending upon the organics. There are surface skimmer attachments for some canister filters; I used these for a time, but small fish can be pulled in, and floating plants can clog them (with or without screens, and this makes them ineffective).

A protein scum is not really harmful, though it can reduce the gas exchange at the surface, so you want to keep it minimal.

If this is cyanobacteria as another member suggested it might be, that is very different.

Byron.

I also agree that it sounds like waste scum/protein scum. I would describe it as "oily dusty water" and I notice it after feeding my fish sometimes. It usually goes away after a few hours for me.