There's one of two ways to fix it. Adjust your power head(s) or return(s) to agitate the surface water more or turn off your power head(s) for a half hour or so, therefore, it can be skimmed off by your overflow, assuming you have an overflow setup. Obviously the latter is a temporary solution which would have to be performed often if it bothers you.
Nevertheless, the debris and oil slick on the surface of a tank is natural, albeit unsightly and is not causing any harm to your inhabitants. I would caution, especially with an open top tank, to be conscientious of accidentally introducing pollution in to your system. I.e. aerosols, cleaners, pesticides, etc. Especially, if you have other people who live with you. Make sure to tell them to be careful.