The 510 is also my first DLSR, itś a great camera at a great price. I havent experienced the AF problem yet, but one word of caution in low light situations. This camera does get very noisy at iso 1600 and even at 800. I try to never go over 400. I would recommend using the flash or maybe purchasing an FL-36 external flash unit ($200), but in an aquarium flash may present itś own set of problems.
No matter what route you take gear wise, i would suggest being very familiar with your gear prior to this event. It will probably take some creative maneuvering to properly shoot this event and fiddleing with your gear and settings is the last thing you will want to be doing.

Good luck