Hi and thanks for the reply. I should have provided more details BEFORE the current situation:

- Inground pool 80,000L, Vinyl, outdoor, 1.5 HP Hayward pump, filter with 4 y.o sand, Hayward heatpump heater, HTC chlorine granules + stabilized pucks, have never had to use algaecide, water has always been clear and clean for as long as the pool existed (7-8 years)

- This year: water was crystal clean after we initially shocked it at the beginning of this season, which here was early April

- All of the sudden, 4 weeks ago, it turned green. Pool store analysed the water, Stab was 50ppm, FC= 3ppm, pH=7.9, TA=10. They concluded that FC, although high, could not operate because of high pH and high stab.

Reasonable enough I thought. So I did what they recommended: Multiple times backwash to reduce Stab, lower pH (sodium bisulf), then SHOCK SHOCK SHOCK as you suggested (I use HTH Ca Hypochlorite 65%), then backwash again. All over a period of 4 days.

- After the 5th day, water still greenish, took sample back to the store: Cya 30 (Perfect they said), FC still over 5 (so unlike you mentioned, FC can be high and water can still be green), CC=0.5, pH 7.2 (perfect), TA=0, CH=250. Their explanation: Because TA inexistent, algae can form easily (really?) so their RX: 1) Add TA+++, 2) Algaecide 60% 1L. Which I did, then water got about 75% better for 2 days and back to the greenish color.

- Another sample to the pool store: FC=still over 5 (off their readings but water still green), CC=0.5, pH=8, TA=60, CYA=30, CH=230, no other metals tested. Their explanation: pH too high so FC becomes inactive. Therefore their Rx: add pH (Na bisulf), then backwash after 3 days..

- Last sample: FC=4, CC=0.5, pH=7.2, TA=10, CYA=30, CH=200, no other metals tested. Their advice: Add TA and algaecide !!! then the vicious and ($$$) cycle!!!

Please forgive me because I want to remind you that
1) I have never had to use algaecide in the past to get my water clear. Ca Hypochlorite has always done the job until they recommended their overpriced 1L for 30$, which I used ONCE under their "advice" and for which the effects (only about 75%, not even 100%) - lasted for 2 days!)
2) The water had always been clean and clear before this last month when this vicious cycle started.

I sure am missing something?

Thank you!