Disinfecting Pool Water

Advice from Dr. Laundry
Q.

Knowing the ratio of bleach to cold water that is safe to spray on children’s toys, I thought it would be a good solution for our child’s pool. We have a blow-up pool that holds a little more than 50 gallons. In the summer in Washington, we have two things to worry about: mosquitoes laying eggs in standing water and a slimy build-up on the plastic.

I figured a cup of Clorox would be just enough to keep the slime and mosquitoes at bay and still be safe for my child to swim in. Well, we added a cup of bleach and it immediately turned yellow. Within 30 minutes the entire pool looked like it was filled with, well, urine. It was truly that yellow.

I have never seen Clorox react in such a way. What could possibly be the reason and is it safe for my child?

A.

Wow, that sounds way too strong! The standard disinfecting solution (3/4 cup Clorox® Regular-Bleach₂ with CLOROMAX®/gallon of water) yields a 2700 ppm (parts per million) concentration. For pool use, we want no more than 1–2ppm. Your 1 cup Clorox® Regular-Bleach₂ with CLOROMAX® would have been enough to treat a 1,667 gallon pool.

For your 50 gal pool, you probably should have started with only ~2 teaspoons to get the 2 ppm level.

So, the yellowing was strong bleach reacting with everything in the pool. I would throw it out and start over. Note the last line above! The smell would have kept me (and hopefully your child) from entering the pool.