the modeling light should help out a lot since the Cube's stock lighting is not that great for photography to begin with... for macro shots of the fishes, i find it best if the aperture is closed up to atleast f8.0 but you probably already knew that... but it becomes even more important when shooting wiggly fishes...
by studio light, do you mean flash...? i think both modeling light and flash should work, but i think i prefer flash... try it out and let us know the results... i don't have modeling lights... i don't know how powerful your studio lights are, but one thing i noticed with my Canon Speedlites is the spotlight effect... my tank is only 21 inches long and 19 inches high, but if i put the Speedlight on one side of the tank, the spotlight effect is so great that it would not allow me to shoot on the other side...