Have you had the Southdown for three months, or the algae problem? 80 lbs for a DSB would mean a pretty small tank (20-30g), or do you not have a DSB? What is your nitrate level? High nitrate is fertilizer for algae. If the DSB is doing its job, it should be near zero. Any chance you are overfeeding? That is a common source of algae blooms. During phases of new tanks algae blooms are normal, but they are very temporary and do not cover everything. If yours lasts or is out of control, I would bet on water conditions, espectially the nitrates or phosphates. Good luck.