I'm Baffled

[Jack Sparrow]

OK, I shock my pool (18,000 gallons vinyl) every Sunday night with hypochlorite. I keep Trichlor 3" tabs in a floater at all times, anywhere between 1-3 tablets. I check my pool daily for chlorine levels making sure all is correct and that it never falls below 2ppm. My other numbers are all fine:
FC- 2.0
TC- 2.0
PH- 7.5
Alk- 115ppm
Hardness- 270ppm
CYA- 40ppm

And what do I wake up to a few days back-- an algae bloom.
Any clues why this occured?

[Guest]

Shock levels should be 15ppm and above and need to stay at that level over night into morning to adequately kill an algae bloom.

Hope this helps.
P.S.
I'd also watch the CYA level in the near future since you are using Cal hypo and pucks because they add to the CYA level which in turn prevents chlorine from working or needing more chlorine to do it's job hence eating more hard earned money you make. Just an FYI.