For culturing either genera is to insure strong enough waterflow in an open tank (keep no rocks or other objects on the tank bottom) so that the algae colonies grow in a constantly tumbling ball! This can be achieved or enhanced by sealing a small lip of glass or acrylic 5-8 cm wide just below the water surface (the length of the aquarium side wall) where the refugiums feed water jets into the tank. Thus, the incoming water is diffused along that slightly submerged lip and is forced to shoot across the tank surface, rather than at a deeper angle underneath. This causes a circular/rolling current of water (like an eddy in a swimming pool) to flow across the top of the tank, down the far side-wall and underneath across the bottom before coming back up again. A small starter colony of Chaetomorpha or Gracilaria will then begin to grow in a symmetrical ball and enjoy not only the benefit of evenly distributed water flow, but also the cyclic exposure of all parts of the colony to the bright light as the surface as it tumbles around!