To Shock or Not to Shock?

[cschnurr]

At pool opening in early May, I topped up CYA to 70ppm per Aquarite SWG recommendation (60-80ppm).
Last Thursday (Day 1), I measured CYA = 55ppm, so I topped it up a 900g bottle CYA which should have put me up around 65ppm.

On Saturday my readings were FC=3ppm CC=0.
Anticipating a family party on Sunday, I increased the SWG output. On Sunday before the party, I had FC of 5ppm (didn’t measure CC).
Since Sunday, I’ve noticed the water has a green tinge, does not sparkle, and there is a small trace of what could be algae on the bottom of the pool.

Tuesday, (Day 6) I measured CYA and it was 80ppm.
It’s a subjective test so I’m thinking the delta between my expected 65 and the measured 80 is the result of measurement error of each result.

I vacuumed the pool (I didn't backwash filter yet) and now have the pump running 24/7, no sign of any CC, and when I turned off the SWG overnight, the FC is maintained at 5ppm. The water has visually improved slightly, but I still have a small trace of what could be algae on the bottom.

My current readings are
FC=5
TC=5
CC=0
TA=100-110
CH=250
CYA=80

My theory is that the CYA I added ‘tied up’ existing chlorine just as I increased the load on Sunday which may given algae an opportunity.

I will vacuum again and backwash the filter.
What do you think, should I shock, or let the the pool clear itself?

[aylad]

Shock it. It's possible that you already had a bloom starting before the CYA addition, or that the party introduced more of a bacterial load than the superchlorination was able to cope with, but if your water is green tinged and not clear, you definitely need to shock it. Vacuuming the algae up doesn't really get it out of your system unless you kill it first--even if you backwash, it doesn't all wash out and it only takes a little to keep the algae growing in your pool..

Janet