Clarification: "shock" is just regular chlorine. "Shocking" just means 'adding more chlorine than usual!'

Mustard algae tends to be very chlorine resistant, and tends to reoccur. Most algaecides are ineffective against it. There are several options, but most have undesirable side effects.

1. Very high levels of chlorine, typically 30 - 50% of your stabilizer level. (If your stabilizer level is 60 ppm, you'd probably need 20 ppm to get rid of mustard.)

Contrary to what many think, you CAN swim with chlorine this high, but it is hard on swimsuits and may dry your skin.


2. Heavy duty application of high concentration copper algaecide.

The problem with copper is green, greenish-blue, or bluish-green: copper stains pools and hair.


3. Monochloramine, produced in your pool by raising the pH, adding products like Yellow OUT, and then 'shocking' the pool.

But monochloramine is VERY irritating to people, and is somewhat hard to remove.


4. High levels of chlorine (15% of your CYA level) + low levels of phosphate (< PO4)

Currently, this is the option I'm testing locally and am most hopeful about. You do have to use another testkit, and you have something else to buy, but the cost has been lower than I expected and the result -- so far -- has been better. This is the only option that has no bad side effects on pool users.

No matter which way you go, you need to manage your pool chemistry accurately, which is why we recommend the Taylor K2006 kit. See http://pool9.net/tk/

If you want to go with the low phosphate option, you'll ALSO need the phosphate kit, shown on that page. With a 10K gallon pool, a single quart of this product should be enough:
Clorox Phosphate Remover
The Kem-Tek product we'd been recommending is being phased out, and replaced by the Clorox line. But for now, shipping times will be slower.

Get 2 quarts, if you want to maintain low phosphates all summer. And keep in mind, low phosphates do NOT kill algae; it simple makes the algae slower growing, and more susceptible to chlorine.