An Olympus E510 not the greatest, not the worst. A better question for taking pictures of your fish would be what flash do you use and how. I am still using my lowly on-board /sigh hope to remedy this situation soon. With an entry level DSLR and two off-board flash, you can take some fantastic tank shots, with practice and some know-how. You don't need a $3000. camera to do so. One would be nice, if you can afford of course.
Peace
Mark