As ClamKnuckle mentioned, an ro
unit is very important in controlling algae. If you are using regular tap water, it's possible you are just constantly feeding the algae with all the crap that's in tap water.
When you say your lfs
says you water tests OK, what are they testing for and what are the results? They may say a nitrate level of 40 ppm
is OK, but that can feed microalgae and also stress out your fish.
Also, how often do you feed you fish and how often do you do water changes?
With a little more info I'm sure you can get better answers to help you with the problem your having.