I would be looking at reducing the nutrient load perhaps with plants and tilapia, but as suggested by Dave, I would have the water tested for pollutants including e. coli and heavy metals, especially if you are considering eating fish from it. Once you know the water is not a hazard, you could consider installing a drain pipe and pumping enough water to get some flow through, as long as this will not cause pollution downstream. Building an emergent wetland downstream might help improve the water quality leaving the pond and provide wildlife habitat at the same time.
