How to remove yellow stain at the bottom of the pool

[andrewkuo]

Here's some information of my pool
* 24,000 gal
* concrete with plaster
* cartridge filter
* 1&1/4 hp pump

There are some yellow stain at the bottom of my pool and I am not sure what causes it. I've try to shock the pool and use the brush to brush it. Nothing works. Can any shed some light? Thanks!

Regards,
Andrew

[PoolDoc]

Sounds like mustard algae. If it brushes away, and then returns in a day, it IS mustard algae.

Do this:

1. Read the Best Guess page in my sig; to eradicate mustard algae you have to hold chlorine to the higher shock level.
2. See if you can find this kit http://www.walmart.com/ip/HTH-6-Way-Test-Kit/17043668 in your area. If you can, test your CYA level.
3. Failing that, get a cheap OTO / phenol red kit.
4. Until you know more about your pool's chemical levels, run your chlorine 2x higher than whatever you have been running.
5. Read the test kit info page; order a K-2006
6. One option to make it easier to control mustard algae is to use borates at 50+ ppm. That's a lot of 20 Mule Team Borax, which is only 11% effective borates. But, it's something you can do quickly, if you want so