Brown spots on the surface of a plaster pool are a sign of heavy metal staining and the likely culprits are iron or magnesium. The metals come into the pool through the water source. According to service company All Pro Pools Inc., metal staining is a sign the pH balance is acidic and the water has become corrosive, and the corrosion caused enough damage to pipes to dump metal into the water. Once you correct the problem, removing the stains is not difficult.
Locate the source of the metal that is infecting the pool water. This is paramount to correcting the problem and might require professional service for your system. It will do no good to remove the staining if metal continues to contaminate the pool, because the staining will return.
Fill an old sock with 1/2 lb. of pH reducer. Tie off the open end of the sock to create a bag for the chemical. Drop the sock into the water near the stain.
Grab the sock with the pool pole and push it down until it settles on the stain. You need to leave the sock in place for five to ten minutes. Either prop the pole to hold the sock down or stand holding the pole in position.
Lift the sock off the spot, if it is gone, then treat the next stain with the same sock. If the spot is still visible, repeat the process. Move the sock around the pool until all spots disappear.