We let coralline algae grow on two walls on our tank and have noticed that there is a definite line above which the coralline doesn't want to grow. It is so obvious it almost looks like we clean down about 5" and stop. On another thread I noticed someone wrote that coralline doesn't like strong MH lighting. We do have 2 250 MH lights on a 140 tank. Would they tend to retard growth to that depth?