I am having a problem with a green algae ring around the outer edge of the pool . It seems to have started after changing the pump ( down graded from 1.5 h.p. to 1 h.p. ) choline level and p.h are in range and pool water is crystal clear . . The pool is a 15000 gal., above ground pool and I am running with a d.e. filter . Also I have changed the D.E. atleast 3 time this summer . I also have discovered a couple of leaks in the liner which I have repaired . The liner is 12 years old . This green stain comes off with a little elbow grease, but comes right back. Any help would be great.