riffin said:
We chlorinate with a floatting chlorinator which goes through @ 3 3" tabs a week. we add chlorine when ever its low. we test the water with test strips. I live it CA. Modesto area. the central valley.
In your 15,000 gallon pool, 3 3" Trichlor tabs/pucks (assuming 8-ounce weight pucks) would add 11 ppm to Free Chlorine (FC) and 6.7 ppm to Cyanuric Acid (CYA). With a cartridge filter, you don't backwash, so with minimal splash-out one summer season of 6 months would increase CYA levels by 174 ppm. Unless you changed the water over the years or diluted with winter rains, your CYA level is probably through the roof and makes your chlorine very ineffective. At 174 ppm CYA you would have to have an FC level of 13 ppm to prevent algae growth.
So I agree with duraleigh that your pool is probably on the cusp of an algae bloom with your chlorine just on the edge fighting algae and creating lots of organics (dead algae) that is getting caught in the filter. You should strongly consider getting a good test kit such as the one at tftestkits.com
here or the Taylor K-2006 (not the K-2005) which can be purchased online from several sources (the TF-100 at tftestkits.com has a more logical amount of reagents and also tests CYA down to 20 ppm instead of 30 ppm in the K-2006).
Unfortunately, if you pool's CYA is really high, you need to do a partial drain/refill to lower it. In the meantime, you can help prevent the algae growth by using PolyQuat 60 algaecide, but I'd first shock with chlorinating liquid after you get water test results (though I suspect you won't be able to shock high enough to quickly kill the algae because the CYA will be way too high -- the partial drain/refill is the most likely recommendation, but let's see the results first).
You cannot use only Trichlor pucks/tabs when you have a cartridge filter, unless you frequently dilute the water somehow. You should be using chlorinating liquid or unscented bleach instead. If you have a plaster pool, then a CYA above 150 ppm could degrade the plaster. So knowing the CYA level is important not only for proper disinfection, but to prevent problems with plaster pools as well (see
this link for example).
Richard