In 2016, the sea trout population in a city stream of Helsinki was in trouble. A large deliberate leakage of oil from a garage in the nearby industrial area contaminated the side stream and threatened the whole population. Storm water running to the stream was deteriorated by smaller accidental oil leakages and loading of hazardous substances from the nearby busy streets. Thus, a delicate stream ecosystem in the middle of the city, a densely built catchment area, and very little space for any management structures to clean the storm waters. What to do?