Hi. I have just moved into a new home with a large 12m pool and 90 000L. The pool is very old and the floor and walls have extreme ruff patched. There is further algae that grows on these ruff patches. I am not sure which colour it is though. Since we moved in two weeks ago a pool guy came around and gave the pool a boost with 1 container of Blue 52 (for 50 000L pools) and put entire bottle in. He further put in a bottle of Zodiac Alcidine again for 50 000L pools, 4 bags of HTH Stabilizer and 400g of Soda Ash. The pool water is nice and clear but the patches is still anywhere. At first glance it looked dark brownish / black. The past two days I have been brushing the patches with a hard algae brush and managed to get a lot of it off, and the remainder now seems to be a light brownish colour and is still stuck. I have also bought a 4 in 1 tester kit and tested the water levels. Chlorine level was at 3 and PH level at 7.2. (not sure about other levels). What more can I do to get rid of these patches and keep it from coming back. Please HELP