Concrete inground plastered pool.
FC 10 ppm CC .5 ppm TC 10.5 ppm CYA 35 ppm CH 675 ppm TA 140 ppm PH 7.7 ppm
Pool was re plastered in April this year (i.e. 4 months ago). A couple of weeks ago I started noticing strange black stains:
Looks like it is getting worse.
I doubt this is algae, since stains do not come of from brushing (even with a metal brush).
I have not tried to rub it with a vitamin C (btw, where can I get pool vitamin C tablets, and how exactly to try that?) to understand whether it is caused by the iron / copper binding to the plaster. The water is clear, it is municipal, and the plaster looked ok for 4 months.
I am not certain whether a 4 months plaster could start exhibiting some bad chemistry. It was supposed to be a regular diamond dust plaster, I would expect to see this problem earlier if the plaster itself was bad (but let me know what you guys think).