There has been some discussion in the past on this forum about the ability of an extended algae outbreak to break down the CYA. I think chemgeek or waterbear put a detailed explanation of why this happens somewhere, but in layman's terms, your algae bloom has probably degraded your CYA to zero. Given this condition, you're going to lose as much chlorine to the sun as to the algae, so you're fighting a losing (and expensive) battle. I would add enough CYA in one dose (it'll take weeks if you just use trichlor) to get your pool up to about 20 ppm, and in the meantime raise your chlorine to shock level and keep it there by testing and adding more every few hours like you've been doing. Keep in mind that your chlorine is going to be much more effective if you'll add it at night because the sun's not consuming it, and that it'll take a week or so for your CYA to completely dissolve, so you're going to have to be pretty consistent with your testing and bleach additions to clear up the bloom.

Janet