A flash definitely helps. So does waiting.
I took this one with the off-camera flash
Of all the pics I've taken, this is one of my favorites, partially because of the difficulty factor.
This one was taken without flash. I sat in front of the tank for about 30 minutes waiting for this bugger to slow down enough for me to take the pic. My estimate would be that the shutter speed was around 1/60. The only reason I got it that fast was I shot at ISO400 and then used a noise-reduction action in Photoshop to get out the graininess.
So, there is no secret, but an off-camera flash, fast shutter, and patience all pay off, especially when you use a combination of the above.
Patience is free.