How to SLAM a pool

What SLAM is

SLAM stands for Shock Level And Maintain. Instead of dumping in a bag of shock and hoping, you raise free chlorine to a level set by your stabilizer (CYA) and hold it there, testing and topping up through the day, until every bit of algae and organic waste is gone. It takes anywhere from a couple of days to two weeks, depending on how bad the water is. The method was developed and popularized by the Trouble Free Pool community.

When you need one

• You can see algae, or the water is green or cloudy.
• Combined chlorine (CC) is above 0.5 ppm.
• Your pool loses more than 1 ppm of free chlorine overnight. See the overnight chlorine loss test.

Before you start

• Get a FAS-DPD test kit (such as the Taylor K-2006C or TF-100). Strips and color-match kits can't read chlorine at SLAM levels.
• Test CYA. It sets your target. Between 30 and 70 is workable. Below 30, add stabilizer to 30 and use 30. Above about 70, replace some water first, or the SLAM takes a lot of chlorine.
• Bring pH to 7.2 to 7.5. You won't be able to test pH once chlorine is above 10 ppm.
• Buy liquid chlorine: roughly 10 gallons for a pool under 20,000 gallons, 15 for a bigger one. Liquid only: cal-hypo adds calcium and dichlor adds CYA, and a week of either causes new problems.
• Turn off the salt cell and any tablet feeder, clean out debris, and make sure the filter works.

Your SLAM level

The SLAM level is 40% of your CYA, using 30 if your CYA is lower than that.

Use the chlorine calculator to work out how much liquid chlorine gets you there.

During the SLAM

1. Raise free chlorine to your SLAM level.
2. Run the pump 24 hours a day.
3. Test and top back up to the SLAM level three or four times a day. Waiting at least two hours between tests lets the chlorine mix and gives a reading you can trust.
4. Brush the whole pool every day, walls, floor and steps, and clean the filter when its pressure is about 25% above clean.
5. Keep going. Letting chlorine drop below the SLAM level gives the algae a chance to recover, and that's the most common reason a SLAM drags on.

How you know it's done

All three, on the same day:

1. Combined chlorine (CC) is 0.5 ppm or less.
2. The overnight chlorine loss is 1 ppm or less. Test after sunset, again before sunrise, and add nothing in between. How to do it.
3. The water is crystal clear, with no visible algae anywhere.

Then let chlorine drop back to its normal target on its own, turn the salt cell back on, and test pH again.

Mustard (yellow) algae

Mustard algae hides in equipment, toys and swimsuits. Pass a normal SLAM first, then hold chlorine at 60% of CYA for 24 hours, and wash anything that was in the water.

Can you swim during a SLAM?

Trouble Free Pool's guidance is that swimming is fine as long as you can clearly see the main drain at the deep end and free chlorine is no higher than your SLAM level. If you can't see the bottom, stay out: you couldn't see someone in trouble.