Hello, bought a used 2 person 130gallon hottub. Ran spa cleaner line flush for 3 cycles, emptying the tub each cycle. Wiped the shell down with bleach. Filled it for my final fill, balanced the alkalinity, pH, hardness, then added my bromine granules and tabs. Levels were good, bromine bank solid, added my shock (MPS) then the next morning a few black spots formed on the walls. I drained it, and they don't wipe off with bleach and a paper towel, but they wipe easily off with muriatic acid and a paper towel. Algae? Iron? I am using well water that is prefiltered before the tub. I did 3 cycles with the line flush, and the only sludge of any sort were these red foam/flakes that I'm assuming is due to the high iron. Do I have an algae issue or a metal issue? This is my first tub, so I'm a rookie