Wow gone a few day days and the thread blows up.. there's is no reason to pour concrete in the bottom of the basin a way over kill. If you are really worried about settling add some crush 411 in the basin bottom and compact bout 2-4 inches worth. I hardly ever use perform basins I custom build all of mine using liner cinder block and heavy abs grating. I build these water features every day I've just got done building a 1250 pound bubbling rock feature. I have bubbling features that have been in for years and haven't settle. I've have a 2500 pound bubbling granite boulder going in a few weeks. If you search my threads there is a pic of a 1 ton bubbling rock that I've posted on here and has never settled as well. Pm me if want.
Turner Landscaping, LLC