Excess protein without surface agitation causes it. I seem to get this on my tanks that I run pre-filters on. There are several ways to get rid of it. You can increase surface agitation by letting the water level down a little if you have a HOB filter. Add an air stone. Or do what I do, and run a protein skimmer like the Ehiem 350. I have mine on a timer and turn it on for 10 minutes a few times a day. They are easy to maintain. I would rather do the maintenance on them then having to listen to an air pump or a waterfall.
This isn't rocket surgery