Pitting and scaring in plaster pool

[catagory5]

I have noticed this summer and last summer that my pool has some 'etching' or little holes that look like something has eaten out of it. Most of these pits or scars have some type of blackish algae in them that is very hard to get out. I don't have alot of this but there is a few spots around the pool that I can see when I get in the water with googles. I asume the pits were caused by something like chemical imbalance and then maybe black algae thought it was a good hiding place for itself. Am I right? What causes etching and pitting and how can I repair it and get rid of that black algae looking stuff?

Thanks

My last set of numbers

FC 6
TA 70-80
ph 7.7 (I usually keep it from 7.4-7.8)
CH 290
cya 80

 

[reebok]

could it have been low calcium in the water leeching from the plaster? your ch seems ok now though. did you ever have to add any calcium? how long have you had the pool (in other words, were these spots there before you got it)?
as far as the algae, I got rid of my black algae when I saw a post here that said to rub a trichlor puck on the spot(s). I did it for 2-3 days and it's completely gone now. you may have to break the puck into pieces that will fit into the spot so you can rub it really good.

 

[MJCP]

"If" you have the same situation I do -
I suspect mine was poor preparation before the previous owner painted our pool.
It started out like yours, small black spots usually tiny, I started seeing blue paint flecks in the pump basket after vacuuming. It got worse each year and now I am planning to repaint next year.
Now I have complete chunks of paint missing and starting to see some deep pits in the concrete.
After reading up on proper prep for my paint job, I am almost convinced that in Michigan at least it will be impossible to get the perfect conditions for painting, and I may be faced with having to repaint every few years.