ive been trying to clean all the algae out of my old 55 gallon tank all day....scrubbing with a sponge, and rinsing with the hose....i dont want to refill a smelly, algae infested tank which i plan to grow corals in......i thought i had won the battle, seeing no algae, but after closer inspection, a very thin layer remains.....it seems impossible to get off with the sponge, but is still driving me nuts (it smells like low tide).....how can i get rid of it?