BD and I are thinking the same thing. Have you always been using Tri-chlor tabs or Di-chlor granules to chlorinate your pool? If you have, you almost CERTAINLY have a very high CYA and a chlorine level of 6-7 isn't going to keep it algae-free.

We don't recommend using non-chlorine sanitizers when the answer for you is very simple:
1) You need to determine your CYA(stabilizer) level. That means you need a good test kit. Our first choice is the Taylor K-2006/K-2006C ( see http://www.poolforum.com/pf2/showthread.php/14994 )
2) You need to raise your chlorine level (FC--Free Chlorine) to the recommended shock level for your CYA level (which I suspect is over 100ppm). (Use the Best Guess table to determine that:
http://poolsolutions.com/gd/best-gue...ine-chart.html )
3) You need to run your filter/pump 24/7 till you can maintain the shock level for 24 hours, and you need to brush and vacuum your pool to waste every day.