My inground pool has a marcite finish that is a few years old and is in very good condition. However, there are a few blackish colored spots on the bottom, in the corners and near the ladder. I have tried using a Siphoning Device on the end of a vacuum pole with some acid solution, but nothing has happened. I can’t rub the spots off. Can this still be algae? Thanks.

The fact that the acid solution had no effect increases the likelihood that it is algae. Black algae, in particular, can be difficult. Fortunately you only have some spots and that can make treatment easier. Your Marcite finish allows for more direct treatment. You have several options. Brush the surface to help expose the algae. Place a 3″ trichlor tablet on top of a spot and allow to remain in place for at least a few hours. The tablets can affect some masonry finishes, so either test it on an inconspicuous spot or verify treatment suitability with the finish contractor. If improvement is seen, repeat elsewhere, as necessary. Another option is the use of a Granular Trichlor to sprinkle onto the spots. Shut off the filter first, so that the granules sink straight down. Use a brush to do the positioning. Another method is to attach a 3″ trichlor tablet to a gadget (available in pool stores) on the end of a vacuum pole. This can be used to rub the trichlor onto the spots. NONE OF THESE OPTIONS ARE SUITABLE FOR OTHER THAN MASONRY FINISH POOLS – DO NOT PERFORM ON VINYL LINED OR FIBERGLASS POOLS.  In order to help keep the algae from returning, you should consider the use of a Polymer Algaecide. This product is effective against many resistant types of algae, doesn’t foam and is non-metallic. To better assure proper overall pool water chemistry, visit a pool store that has a very reliable, professional lab such as a WaterLink SpinTouch Lab, rather than a less accurate test kit or strip reader.  If this website was helpful, in answering your question, please consider joining our E-Letter Mailing List.  I hope that this recommendation works out for you.