tom, i have a 37 gal tank with petrified rock glued together with gravel in center. unable to remove these for cleaning.quite a lot of bba on these rocks. tryed to clean with brush ,to no avail. did 2 three day complete blackout periods with no luck. plants in tank,for the exception of crypt willsii,which is a slow grower have no bba on them. very healthy. do i need to do longer blackout period? this stuff is a unbeleivable pain in the butt. regards,cornhusker