trichlor tablet question


TFP Expert
LifeTime Supporter
Feb 23, 2008
Denton, TX
Since switching to BBB, my CYA has started to normalize. This is a good thing of course because I can start to bring my FC levels down to levels that won't bleach out swimsuits quite as bad. However, we have had lots of rain recently and, as a result, the CYA will continue to decline due to the water replacement from storm water going in the pool. Now, since the need will arise to bring the CYA back up at some point, I was wondering if I could use my leftover trichlor tablets to do this. Is someone able to tell me, for example, with 1 3-inch tablet of trichlor in 10,000 gallons, just how many ppm would my CYA be raised assuming that the entire tablet dissolves?

And YES, it is 4:15 a.m. here and I am wide awake thanks to yet another dryline driven storm blowing through and dumping a TON of hail on us. Only marble sized this time but the sirens were blaring nonetheless. Such is life in north central Texas. Quite a few strong convective systems lately. Glad my 2 year-old isn't afraid of the storms and the sirens yet!



Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Hi, Craig,

What's a "dryline" driven storm? I've not heard that term before.

Chemgeek and others here know the exact answer but I can roughly estimate while they're snoozing. Look on the tabs container and see if it lists the % CYA (If not, you can sorta' deduce it by subtracting the chlorine from the total)....I think it's around 10% or so but I've never used tabs.

Then simply take the weight of the tabs you intend to introduce to your pool and multiply that percentage. So, if you put in 5 lbs of tabs and it's 10%. you're introducing .5 lbs of CYA in the pool.

Then look at Jason's calculator to figure how much ppm that will raise your pool.

After all that, I can tell you that using a few tabs in your pool to chlorinate will not add too's the continued usage of tabs year after year that causes big problems.


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
Trichlor tablets come in various sizes. Assuming a 7 oz tablet in 10,000 gallons you are adding 4.8 ppm of FC and 3 ppm of CYA. If your tablets are some other size you can use the "Effects of adding chemicals" section of my Pool Calculator, see the link in my signature, to calculate what the numbers are for your tablets.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
257WbyMag said:
This is a good thing of course because I can start to bring my FC levels down to levels that won't bleach out swimsuits quite as bad.

Jason answered your question regarding Trichlor tabs and FC, CYA. Regarding bleaching of swimsuits, it isn't high FC that causes this, but rather the amount of oxidizing chlorine known as hypochlorus acid that causes this. Roughly speaking, it is proportional to the FC/CYA ratio so a higher FC with higher CYA will not degrade swimsuits any faster. If there were no CYA in the water, such as with most indoor pools, then a higher FC would indeed degrade swimsuits faster and even low FC degrades them faster than pools with CYA. My wife's swimsuits degrade probably around 20 times faster in the indoor community center pool that she uses compared to our own outdoor pool with the difference being that the indoor pool doesn't use any CYA.

What is the FC and CYA level where you are noticing bleached out swimsuits? Do you rinse out the swimsuits after you use them? If not and if they stay damp, then in that situation a higher FC (independent of CYA) could cause more fading not so much due to any change in rate, but due to a larger total capacity of chlorine that would continue to oxidize the swimsuit over a longer period of time (i.e. the chlorine won't run out as quickly). After we are done using our pool for the day, we rinse our swimsuits in mild soapy water in a sink.