A few questions:
-- I assume the chlorine tabs are trichlor?
-- What is your cya reading?
-- What is shock plus - meaning ingredients. I suspect it is dichlor.
-- What type of pool (vinyl, concrete, etc.)
I'm pretty sure I know what your problem is. Those tabs are most likely trichlor which is stabilized, which means they add cya. Dichlor also adds cya. Over time, cya builds up and you have to run higher chlorine levels as a result, which most people don't realize. Thus, algae takes over.
Repost with answers to these questions and somebody will help you from there.
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