I usually use my 24-105 kit lens with my 5D mkII to take the fish pictures. ISO I like to keep at 400 for most aquarium pics, sometimes go to 800 if I need a bit more light. I just really hate noise in my images so I keep ISO as low as possible. Noise level will vary greatly at different ISO's for different cameras. F-stop is as low as the lens will go, f4-f5.6 depending on zoom level. Not great as far as light goes, but going any faster with a different lens there makes depth of field so incredibly shallow, not really worth it IMO unless you're looking for that kind of picture. Shutter I use 1/200-1/250 usually, unless the fish is swimming really fast that's usually good enough to not get motion blurring. Shutter speed is key, just remember whatever your other settings are, if you use too slow of a shutter speed and there's motion blurring there's no way to ever fix that. If the picture is dark/underexposed from too fast of a shutter speed it can be lightened in post-processing. Lots of tradeoffs with different camera settings :) Lower ISO - higher quality image but less light captured, higher ISO - more light captured but image quality degraded with noise, lower f-stop number - more light let in but shallower depth of field, higher f-stop number - less light let in but greater depth of field and more of the image is in focus, lower shutter speed - more light captured, but can get motion blurring from moving subjects, higher shutter speed - less light captured but 'freezes' motion on moving subjects. Its all about finding the correct balance for you and your camera of those three settings.
With faster fish, you'll have to 'track' them to get the best results, don't wait for them to swim where you are but constantly stay right in front of them as they swim. Watch their patterns, sometimes they'll swim slower in different areas of the tank, or you'll notice they pause in some section of their swim pattern, or slow down enough to get a good pic right before they turn. As you keep moving the camera to keep them in the frame, it helps with motion blurring on the fish, plus focusing. Digital cameras have 2 steps to the shutter button, halfway press focuses, full press takes pic. The delay between the two has gotten very quick with cameras now, but its still there. So if you're stationary taking a pic of a fast moving fish, by the time you fully press the shutter and the camera focuses and takes the shot, you're often left with just a tail in the pic :)