I'm doing a drain/refill due to excessive CYA (seduced by tabs in a floater... oh well) and CH higher than you'd like. As my wimpy sump pump strains to perform this herculean task (I estimate 4 days to drain 3/4 of the water) I'm noticing that the plaster looks kinda dirty. The water has always been clear (at least as long as I've owned this place, about 4 months). A very brief scrub with a hand brush didn't seem to have any particular effect, although I admit it was kind of a token effort.
Should I plan to go at the plaster with something while the water level is down?
Probably not a detergent, but maybe dilute bleach would be okay?
Or is this more likely to be a scale-type problem?
Or does plaster always look like this? I have no idea how old the pool is.