What are the brown spots on the surface of my water?

[tjtax]

These came out of nowhere a few days ago and have no idea what is causing this.

 

[DED65]

Do you run a skimmer? It looks like protein or fat left over from feeding.

 

[taricha]

"scum" like this in my experience is likely a mix of oils (from foods), debris, and some algae like cyano that can form films at the surface and take advantage of the stuff there.

 

[Mr. Mojo Rising]

Seems to be minimal flow in the tank, the debris and rot is able to just rise to the top. Need more flow and more surface agitation to mix that stuff into the water so your filters can filter it out.