A fast shutter speed is pretty much a must for fish. 1/200 or faster. Getting enough light for that fast of a shutter speed is tricky, and probably isn't going to happen in a fish only tank with sub-par lighting. In a reef or planted tank, it's possible... I just set my camera to continuous shutter, 1/200, and manual focus. Out of probably 30 pics, there might be one decent one.
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