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My plaster pool has developed  persistent green-brown algae stains in the last couple of months. Water chemistry (as checked by the pool store) is fine, volume is 17000 gallons, with a DE filter, recently cleaned. I've had the place for 5 years, and in the past such algae was easily taken care of. This strain of algae seems to have imbedded itself into the plaster. No amount of wire brushing can remove it. Algicide similarly seems to have no effect. Any ideas?

Rod 

First you need to determine if the stain is really organic or whether it might be a metal stain. There are a few tests you can use.  Hold a trichlor tablet on the stain for about 10 minutes (use your pool pole to hold it, not your fingers).  If the stain lightens it is organic and should respond to chlorine. You can try placing cal hypo right on the stain to bleach it out or you can treat the whole pool with sodium percarbonate.. 

If the stain does not lighten or gets darker or changes color to black then it is probably a metal stain.  Try holding an ordinary vitamin c tablet on it.  If this removes the stain then you can treat the pool with ascorbic acid to remove the stain.

If that doesn't work put some dry acid (sodium bisulfate) in a gym sock and hold it on the stain for about 20 minutes.  If the stain lifts or lightens then you probably need to acid wash the pool to remove the stain. There are some products on the market that can remove this type of stain withouth acid washing the pool but they require several weeks to work and the pool is not swimmable during the treatment. 

If waterbear's suggestions don't work, the stain may actually be the gunite starting to show though the finish coat.

Mike 

Thanks for the suggestions. I'll try the tests later today. A few weeks ago, I did try rubbing the stain with a 3 inch chlorine tablet for some minutes, but without obvious effect. However I'm pretty sure that its organic, as it occurs where algae has always grown, around the steps and on the wall at the deep end, but in the past it had been easy to brush off. Now, brushing removes about half of it, the remainder still apparently in the plaster. I suspect that the pool has evolved a resistant strain of algae.

You might have a phosphate problem.  There are products to remove phosphates from the pool.  Read and follow instructions.  You did not say what your chemical readings were.  Total and free chlorine, pH, alkalinity, conditioner levels.  You may just need to use products like Yellow Treat by United Chemical or Yellow Out.  It also could be that you had and algae bloom and a calcium fallout thus locking the algae under a film of scale.  You may need to check your chemistry more frequently in the future.  If in fact you do have a metal problem - ie. black stains - a no drain acid wash will work in less time than what was proposed previously - taking maybe about four days to remove stains etc.  A no drain acid wash is best left to someone with a good background in the process.  The key here is knowing if you need  the A/W, getting it started properly  and knowing when to stop the process.  United Chemical in Calif may be able to advise.  I have no connection with that company or any other.

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Ken B