I’ve always wanted to photograph these sinuous creatures… safely from behind a pane of glass that is. What I didn’t figure was how difficult it’d be! The ones I saw were not the least bit sedate, and energetically zipped all over the tank the whole time. The blooming and pulsing motions usually threw off my autofocus and made things a wild blur. Later, I finally settled on a shuttle speed of between 1/50 and 1/100, at f4 to f5.6, and whichever ISO is necessary to achieve the shuttle speed. You could also try continuous focusing, or a burst mode to increase your chances of getting a solid shot.