8+ square feet per starfish should be fine. Bio-diversity is overated in a sand bed and not to be taken into consideration. Those that claim it is important do not understand how a sandbed works in the wild or in captivity. It is mostly bacteria which the starfish and cucumbers feed. The rest can get out of the way. There is not enough food in a small tank for a starfish to survive. A cucumber would probably do all rght in a 4+ square foot tank.