Green algae? Mustard algae?

[mbar]

CYA of 75 is ok, as long as you keep your chlorine in range (5-10 pmms). You say that you shock - but do you shock only once??? You have to be very consistent if you want to get rid of the algae once and for all. You have to test the water 3x a day and make sure you take the chlorine back up to shock, which with a cya of 75 is 20-25ppms. If it is mustard algae, you have to take it up higher and longer, and use polyquat 60 to prevent it from coming back. You also have to make sure that anything that was in the pool is cleaned - bathing suits, toys etc. and then put in when the chlorine is at shock level. You may have algae on the cover which is not getting killed and then introduces the algae back in the water when you put the cover back on You also have to brush the pool, and backwash the filter. I am not sure how you tell the difference between mustard or green algae - I have read that the mustard algae looks like sand, but when you rub it between your fingers, it looks green. In any case you will have to be very consistent with the chlorine levels until your chlorine holds overnight, and then give it a couple of more days at shock level just to be sure Good luck and let us know how you make out
You can also look at this thread - it may be something you would be interested in, about how borates keep algae away:

http://www.poolforum.com/pf2/showthread.php?t=7480