I'm buying a house with a stagnant pool - hasn't run in months and is dark dark green. The plan is to refinish it regardless of what is under there, so I'm not too worried. One contractor is insisting that the black algae cannot be eradicated by chemicals and will come back through the new finish; unless I sandblast the finish down to the (presumably) gunite. For a fairly small pool, the added c cost is $1K. Opinions? Thank you!