Over the last week I have noticed some small rust brown stains on/in the plaster of my pool.
The plaster is dark, so the stains are not easy to see and the plaster is less than a year old, so is still changing slightly itself.
The stains are tiny, less than 1/4" in diameter, round and rust brown in color. They seem to be pretty stubborn and do not come off with brushing. At last count, this lunchtime, I found about 25 of them, spread fairly evenly across a 15000 gal pool.
My numbers are pretty good, I think:
Any idea what this might be?