You can also stir the water, try to get some O2 exchange going, as a temporary solution.
+++EDIT+++
Make sure when you stir, try to get water from the lower part of the tank. I have never done this so that's about as far on the advice I can give you (I'm not sure how often you would need to do this). You might even be able to use a battery child's boat (make sure there is no metal), or some other water moving type of deal.