krill is kind of oily, it would need to be since it's a good source of fatty acids. look at all the krill supplements.not to mention the blubber layer on several krill eating whale s. what you're seeing is probably the the excess coming out in the fish waste. combined with low surface agitation gives you an oil slick.
getting your surface moving would reduce, if not eliminate it and enable it to run through the filter.
replacing some of their krill intake with a good pellet would help too.