If it's recurring green algae (free floating), you probably just need to (a) get a K2006 kit, and use it, (b) master the CYA/Cl relationship per the Best Guess page, and (c) be consistent with chlorination.
But, if it's persistent mustard algae -- which is quite chlorine and algaecide resistant -- you may want to participate in the phosphate trials we're doing. You *still* have to do (a), (b), (c) above, but I'm looking at evidence that, with very low phosphate levels, you may not need the extreme chlorine levels that are sometime required.
We've generally recommended AGAINST using phosphate removers, because they have been sold indiscriminantly, often along side phosphate increasing stain and scale chemicals. But a small percentage of pools suffer from persistent and recurrent mustard algae for reasons we have not been able to discover. With these pools, low phosphate levels may offer effective and fairly economical relief.
If you're interested, the thread is here: http://www.poolforum.com/pf2/showthread.php?21409
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