I have a bunch of purple stains on my pool bottom (probably 20 or so in total). Have been there for at least a month. Could be black algae or could be from fertilizer on the nearby lawn getting into pool. I have tried to scrape of some but my nail and a stainless steel brush do not budge the stains, I read that a good test is to rub a tricolor puck on the stain, but I don't use or have any. Is there a cheap way to buy single pucks? All I see are large packs which I will never use.