Calcium Hardness Low - Are these black spots related?


Well-known member
Jun 7, 2019
New pool owner. Had new coping/tile done last September, winterized with standard winterization kit, and closed pool with pool a little over half full.

Had 2 9000 gallon tankers fill the pool last Wednesday, so it has been a week. Just did a calcium hardness test, and if I did correctly, it took 11 drops to turn blue, which means I am low on calcium in my water and need to raise it up to 200 asap, correct?

Something else I'd like to ask about - my pool robot has been picking up a lot of white sandy material...I assume this is plaster dust from the coping/tile work done last fall? Or is this my plaster being destroyed by the aggressive soft water? How fast does soft water take to start damaging your plaster? I noticed some black spots on the bottom of the pool that I didn't notice when I opened it a week ago. Is this due to the low calcium levels? See photos.

I tested chlorine (2.0) and Ph (~7.9) yesterday as well, and added half gallon of MA to bring the Ph down once I saw it was high. Alkalinity looked to be in the 110-120 range. Please let me know what you think, thank you.



TFP Expert
LifeTime Supporter
Jan 6, 2010
San Dimas, CA (LA County)
White stuff could be anything. If it's sandy, it's most likely sand from the filter or a mess left from tile work. If you were dissolving the plaster into sand, you'd see the pitting on the walls. If you can gather some of it up, test it. Is it gritty or mushy? Mushy could be something from a tree that got blown in. What happens when a drop of muriatic acid is placed on a pile of it? Calcium/plaster will fizz. Sand will not.

11 drops could be 110 CH or it could be 275 CH. It depends on how big your sample was. 10 ml or 25 ml?

2 FC is too low, even for a pool with no CYA. If you have any CYA left, your FC is even more too low. That can allow algae to grow, which is very likely what those black spots are. See Black Algae - Trouble Free Pool to learn how to identify it and treat it.

It's much easier for uis if you post test results like this

FC – Free Chlorine
CC – Combined Chlorine
TA – Total Alkalinity
CH – Calcium Hardness
CYA – Cyanuric Acid (stabilizer)
Borates (if you are using borates)