I have been a pool plaster installer for 13 years. I install Diamond Brite as well as many other pool finishes. The brown and black spots that people often complain about are usually caused when fertilizer pellets are put around poolside planters and some of the pellets get into the pool. Most fertilizers are very high in iron which will stain most any pool finish. I am in no way a rep. for Diamond Brite. I'm just a humble pool plasterer-proud of my craft.Feel free to email me with your pool finish problems and maybe I can help.