Green walls - just can't kill it all

[PoolFinger]

Hello, thanks to all who keep this great forum available. I built a pool a year ago and have been using BBB from the start with one exception. I live in Houston and have a tough time keeping enough FC in the pool through a long, sunny day. For this reason, I run 3" tabs on my chlorinator consistently and deal with the rising CYA through more frequent backwashing (DE filter). So, to my question...

I had a light algae bloom a few months ago (week long business trip). It consisted of a light green coating on the walls. I immediately shocked it with bleach according to the recommended levels from poolcalculator.com (troublefreepool settings), brushed, vacuumed. Things cleaned up nicely, and I followed up with some polyquat 60.

However, I continue to have very light green algae on the walls at times, even with my chlorinator set at max continuously. I have not been able to detect any CC with my Taylor DPD test. Another item - I have rocks around much of the pool wall that are below the water line that I brush with a hand scrubber periodically. My numbers follow;

FC 4.0
CC 0 (not even a tinge of pink in the morning!)
CYA 40
CH 240
TA 110 (pool store says adj alk is 90)
pH 7.5
borates 30
water temp 89

I am assuming that I need to shock again and hold it for several days to kill off the last of the algae, but without a CC reading, how do I know when I'm all clear?

Thanks a ton for the help!

[AnnaK]

You're all clear when your overnight chlorine loss is equal to or less than 1 ppm. Dose the pool after sundown, circulate for a couple hours, test FC. Get up really early in the morning before the sun is on the pool and test again. You can have an incipient algae bloom without registering CC.

I would raise the CYA to between 50 and 60. It'll take a little more chlorine for maintenance but, on the other hand, it'll be easier to hold FC in the pool. You said you do frequent backwashing which means you'll also frequently have to top off the pool and that, in turn, dilutes the CYA concentration.

I run trichlor tabs in my online chlorinator which I have set to 1/4. Not a lot of CYA gets put in but it gives me just enough FC to keep the level from going to zero. I refill a lot because the dogs drag out a lot of water and have to add granular CYA each season. I keep mine at 60 ppm.

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