Thanks for the good advice!

Muriatic acid seems to have done the trick in our spa. We turned the spa off and added a muriatic to the water. Let it sit for an hour. It really cleaned it up. The concentration would have been about .2% for total volume.

A few questions:

1. What is the maximum concentration of mutriatic acid that can be safely used in a plaster pool? (14400 gal)

2. Can the PH be lowered a significant amount for a period of time then raised to normal levels without potential damage to the plaster?

3. Should the DE in the filter be changed or backwashed to eliminate any residual algea?

4. As an aside, to eliminate the possibility of metal staining, will muriatic acid also remove Cu or other metals from the surface of the pool?

Again thanks for the help. Big learning curve for us on this!