Green Layer on Pool Floor (probably dead algae)

[fhorta]

Hello all. I cannot get rid of fine layer of what i think is dead algae on inground pool. About 25k gallons. Have a sta rite 3 sand filter. Plaster/gunite, not really sure if there is any difference.

I have tried raising chlorine level to 10 ppm but not clearing. I haven't really held it there consecutive days, i think i did hold it at 6 ppm two days in a row. I use liquid chlorine, 12%.

Pump has been on for a week 24/7 now and not letting up. I will add some DE periodically. The pump gauge does go up to 14, 15 pressure and I back wash. Green crap comes out. I keep filtering but that stuff still there.

I tried vacuuming to waste. But my system don't have vacuum to waste setting. So i just set it to backwash and manually vacuum. I did that yesterday but this morning floor still the same. After vacuuming, I raised chlorine to 12ppm last night and this morning still about the same chlorine. Floor still same.

I had water tested and was told combined chlorine is about 2.5ppm and i needed to throw in all 4 gallons of chlorine to shock pool. I told lady already at 12ppm and she said its not the ppm, its about how many gallons you throw in???

I have 4 lbs of potassium chloride oxidizer i plan on throwing in tonight. Any ideas, suggestions??Also, plan on changing sand. Its about 9 years old.

Additional info, sorry forgot. CYA is about 25 to 30. I know i neeed to raise it but wanted to clear this stuff first.
Alk is 100
Hardness is 200
Ph before adding chlorine was 7.4-7.6 range. Today is above 8 probly due to high chlorine.
Filter gauge reads 9 psi on fresh backwash. Right now its about 10 psi.
I use the Taylor 2006 test kit.