How much is reasonable

[dawndenise]

OK, Dan, here's my take on this...

First off, this is all subject to a lot of assumptions. Ideally, you'd have test results from your pool service at opening and then again about 24 hours later after the chemical additions had had time to circulate. Even just 2 weeks later, a lot of conditions could affect those results...adding water, splashout, backwashing, and even adding muriatic acid (affects TA, not CH).

But, for sake of argument, let's assume nothing happened in those 2 weeks. Approx. 5 pounds and 9 ounces of baking soda raises TA in your size pool by 20ppm. Let's round that to 5.5 pounds for the sake of our calculations. Your service added 15 pounds. That should have increased your TA by about 55ppm, so one could "assume" that your starting TA was about 78 (78 + 55 from added chemicals = 133 test result). One generally shoots for a TA of 80-120 (with some discussion on whether the higher or lower end is more beneficial to stabilizing pH).

As for calcium... if you have a vinyl pool, you don't need to worry about calcium levels. If yours is a gunite/plaster pool, you're looking for a range of about 200-400ppm.

Some calcium increaser products show a dosage of 5# in a 10,000 gallon pool to raise CH by 40ppm (my pool calculator differs a bit on this, but we'll use 40ppm because that's what's on a number of products). This translates to 10# of calcium increaser raising CH in your 20,000 gallon pool by 40ppm. Your service added 15# so that should have raised your CH by 60ppm. One can then again roughly "assume" that your starting CH was about 108 (108 + 60 from added chemicals = 168 test result). You definitely needed some calcium IF you have a gunite/plaster pool.

The experts on pool chemistry will have to weigh in here on how well-balanced your water is and if there are any other adjustments needed in my calculations...and there probably are some.

As a final note, you may want to give some serious thought to doing your own water testing and chemical additions. It's not that hard once you get the hang of it. A good drops-based testing kit will let you fine-tune your chemicals whenever you want with just the exact amount of what's needed. And, you'll probably pay a lot less for the chemicals you add. Just some food for thought...