My Marcite pool was looking "sick", so I turned off pump, gently applied Jacks Magic iron,cobalt,spot etching stuff. As the powder fell to the pool bottom, you could see the stains disapearing before your eyes!! After 10 mins I turned on pump and then brushed the pool sides, same thing stains went away. I did ISOLATE the pool heater from this process. (not that it was in use). All the metal was collected in the filter. I cleaned the filter AFTER 48 hrs, did not wish to dilute the powder that was ALSO in the filter. Worked fine. MKM were first class suport, as well !!!!