Nice thread going so far. As others have said, upping the ISO will introduce noise into the picture (you can run noise ninja or some other form of noise reducing software over it, but that increase the workflow on your side...which kind of takes the fun out of it). The +/- is exposure compensation and the settings will require some experimentation. Take some pictures and if the picture looks overexposed (i.e. flares or blown), then crank it down.
Most P&S cameras will introduce noise at ISO settings of 200 or more (some will become more visible even before this
). All three Olympus has an ISO setting of 100, 200, and 400. If possible, use the lowest setting (100 in this case), and see how the pictures come out.
You can freeze fish by one of two methods: (1) increase your shutter speed (max shutter speed on the two older Olympus models are 1/800 sec. and for C5050, it's 1/1000 sec.), or (2) use the flash. Brighter lit tanks have less problems, since you have more light to work with the the beginning (you can also set a lower f-stop number to let more light in, and worry about DOF later).
Marcel, as for checking out a digital photography site for fish, you might want to check out www.aquatic-photography.com