fifteen year old 16 k gal gunnite pool S Louisiana no major problems over years since Katrina when had to empty / refill. have spots on pool floor which are not of black algae origin. Have submerged self long enough to scrap the spot surface and unlike upsetting b. algae spots the 1 inch diameter dark spots do not budge. Difficult to see from distance only when standing above in pool. Do these sound like permanent stains?