Pool goes green every July

[Anita]

I have a pool that goes green the same week every year. Usually the third week in June. In anticipation of this, I make sure my chlorine level is on the high side and all my other chems are on point. I also take the pool off the timer and let it run 24/7 beginning the first week of June in hopes this heads off the green. It doesn't. The pool will look a bit cloudy, then the next day it is a deep green. I've had my water tested (both in the pool and from the tap), I've spoken to the landscaper about not using fertilizer anywhere near the pool, I've flocked it - last year I had to drain and power wash the pool. I've had the sand changed in the filter. I really think it is environmental. We do have a landscape shop about a 1/2 mile behind our pool that grows it's own flowers. Could something be blooming at this time of year that is getting into my pool?

This has happened the last (4) years in a row and it is so frustrating not knowing what is causing this. It takes about 2-3 weeks of constant care to get the pool back in shape. Any advice would be very helpful.

[Denniswiseman]

I would suspect that It's the way you chlorinate
Let's have your numbers (CYA is very important) Use a pool store if you don't have a FAS/DPD kit
FC:
TC:
pH:
TA:
CH:
CYA:

And some more info would be helpful
My pool:
Pool chemicals:
My pump & filter:

[Anita]

Thank you for the response, Dennis.

The pool is 20X40 Pebble Tech
Hayward S244T sand filter
Hayward 1.5HP pump
Raypac heater

FC: 5
TC: 3
PH: 8.2
TA: 180
CH: 250
CYA: 20

No phosphates, no metals. These results are from 2 days ago done by a pool store.
We live in Massachusetts - not sure if location makes a difference

We've had (3) pool professionals look at the pool and test results over the years and have done everything suggested: Sand change, run pump 24/7 a week before the usual "turn green", emptied and power washed thinking some sort of bacteria was getting in the nooks of the plaster, flocked. Last year, we had a pool professional flock the pool (he raised the chems that needed to prior to adding the flock) and it took almost a week for it to settle out and even then you couldn't see the main drain of the deep end.

Our best guest is environmental because it happens the same week every season. The pool will be crystal clear, the next day a bit cloudy and then Wham! it is green by day 3 and I'm talking deep dark green. No debris in pool, no algae on the walls.

Any help is appreciated.

[Denniswiseman]

Your Free Chlorine and Total Chlorine is wrong as Total can't be less than Free ( Free Chlorine + Combined Chlorine = Total Chlorine)
If they are the wrong way round then you need to Slam (Shock Level and Maintain) with relation to Chlorine / CYA Chart and Recommended Pool Levels
With a CYA of 20 you need a slam level of 10 of chlorine, use Liquid chlorine (sodium hypochlorite or plain bleach)
Forget about changing sand

[Teapot1]

That 1.5hp pump is a big pump for a 24" filter you could halve that to 3/4hp for that size of filter or fit a speed controller or add another 24" filter. Slower filtration is better filtration and removes more stuff. Pushing too hard with a 1.5hp pump into a 24" filter wastes electricity and pushes dirt through the filter back into the pool so taking longer to clear.
No need to change the sand but maybe a clean of the sand. Yes rough finished pools are harder to look after as the algae finds hiding places and I would run at a higher level of chlorine because of it during these times. How are you chlorinating?