We have a 40,000 gal. in ground pool. In the past 25 yrs. we have had it resurfaced 3 times, about every 7 yrs. . Two years ago we got a terrible infestation of black algae. We spent $100's dollars and back pain. emptied the pool twice and scrubbed our fingers raw. June 2014 we had it resurfaced again ($3300.) We were so worried that we questioned many times if they would be able to remove all the black algae spores? They said yes. They water blasted (the pool was sand blasted before). They put on what looked like two coats of plaster in one day and started filling the same day (took a week before with 3 coats). We were again assured that the black algae was gone and we would have a pool surface that would last for years. They seem to be a reputable company. HOWEVER 9 months and we have black algae in the same areas as before, not as much yet of course. We have been very careful with the chemicals and PH. CAN BLACK ALGAE GROW THROUGH NEW PLASTER? We sent the company a letter of complaint and received back a phone call that said we are to blame. They will not take any responsibility for this. Is there anyway to prove that they were lax in how they surfaced the pool? We are devastated. I haven't been able to find anything online that says black algae can grow from inside out.