You can use delayed aperture feature, if your camera has it. Its the thing they use during night time photography of downtown skylines. If you have plenty of light in the tank, the aperture will relatively close quicker, but you will see, eg, a yellow streak of light behind your yellow tang if the tang moves during this period and you have to use a tripod to take this picture. I have done photography of this kind and belive me it comes out great.
You can also turn off your current flow in the tank for minimum movement.