In the ocean this problem is solved by the to and fro motion of wave action and the angle of the rising and setting sun. Since the second option isn't practical the addition of pumps to alternate the current,such as imitataing incoming and outgoing tides by alternating the every 6 or so hours or the addition of a wavemaking device should expose the hidden polyps to enough light to survive if they are in an area of moderate alteranting flow. If those particular polyps don't survive, others will divide at some point if water conditions are optimal.
Hope this helps,
Ken