I have some strange raised bumps on the bottom of my plaster pool. They range in size from a dime to 1/2 dollar. They can be scrapped off using a pumice stone. What could have caused these and what is the best way to remove them? Thanks

Kinda look like Black Algae, but its hard to tell from the pics.
try rubbing a trichlor tablet on a spot, see what it does.
When you use the pumice stone - does it leave behind any residual or staining?

I'll try the chlorine tablet and see what happens. When I used the pumice stone, the spots get small but you can still see a small mark.

try the tablet on the spots left behind after the pumice stone. Black algae has these roots, the heads are resistant to chlorine but once you "knock off the heads" the chlorine can attack the roots.

It is possible that's what's happening here - how often are you running your pump/filter- and what chem levels are you maintaining? IF you could post a full set of results that would help us advise better.

Black Algae can get started and survive in 'normal' levels of chlorine if circulation is poor - what can happen is these FC dead spots - if there isn't adequate circulation time... esp during the daytime hours when the sun is attacking your chlorine. Do you run the pump during the day at all?

Ann, sorry to say that that looks nothing like Black algae

It does look like some sort of algae clump.

Nail it with cl and I'd bet it just goes 'bye bye'

meatloaf, what CYA and FC levels do your normally maintain?

Do you have test results to post?

Looks like BA pics I've seen...
plus he said he had to remove it with a pumice stone and it left a residual stain. That 'sounds' like black algae from the description.

Test results are,
CL 3
PH 7.5
TA 90
CH 550
CYA 45 (APPROX)

I scraped one spot this morning and set a puck over the remaining spot, there is little to no change and the puck sat there for 8 hours.
Thanks

Well, if the spots were organic then the tablet would have done something I would think.
I'm stumped. Maybe Ted has a suggestion.

Has your pH been above 7.8?...or has it lingered around 7.8?

PH did get to 7.8, but only for a day or two.

High pH + high CH = calcium deposits. Maybe running the ph lower, around 7.2 for a week or so and brushing could clear the problem. See what others think on this.

Also, someone out your way (can't remember who) runs a low Chlorine to CYA ratio - keeps it teetering at or around the minimum. This would create an enviornment for a all kinds of algae, even black.

I think I figured out the problem. Calcium deposits, they seem to scrape off with little or no residue. I'm going to try to keep the PH @ 7.4 or below for a while and see if that helps.
New numbers are:
CL-3
PH-7.4
TA-90
CH-590
CYA-50
TEMP-82*

It may be worth trying a de-scaling product to see if there is some improvement. Your CH may be the culprit (or a big contributor). Is your tap or well water hard?

Tap / Fill water tests @ 250. I've been here for 5 years and the pool has never been totally drained, just partial draining to lower CYA. I think during the winter would be a great time for a complete drain, acid wash & refill.
I did purchase a bottle of BioGaurd scale inhibitor and added that yesterday.

With your fill water that high, and the evaporation rates that you see, your CH will build again quickly. Have you considered R/O (Reverse Osmosis)? There is a good company out near you that I might be able to recommend, if you wish.