I'm about to post the "3 of 3" section of the posts "Avoiding image blur". That part will explain in very simple terms about camera settings. For now, learn how to adjust the shutter speed and try to shoot with a speed of 1/30 or faster (1/60, 1/125...)
External flash is very useful but if your tank is brighly lit you may not need it. For shooting fish, especially fast moving ones, a flash is almost a must though. You do need external flash because the on-camera flashes will force you to angle the camera to avoid reflections. Look at aftermarked cables and flashes - they are much cheaper than the original manufacturer's. Make sure they specifically say they are compatible with your flash.
When or if you get an external flash you may want to soften the light by using a diffuser in front of the flash. The cheapest/easiest to get/use looks like this
- just a plastic "box" that fits snugly on the flash.