Working at a aquarium In Australia seems hard to get to but a pet shop is far easier as usually they have no real knowledge while I have found many aquariums ( In Victoria) have employees that know what they are doing...
Back many years ago, i got my job at a lfs by doing a weeks work experience there. I don't know if school kids still have to do that, it might not be compulsory anymore. Anywho, back to the point, the whole idea behind work experience is that you get a 'feel' of what it's like to work in a given field and in exchange, your employer gets cheap labour for a week or two. For my first week there i didn't even serve any customers, all i would do was water changes, clean the floors, wipe down cover glass, move big bags of gravel around, etc, etc.The kind of things nobody wants to do but have to be done. As boring as it sounds, it was a foot in the door and i needless to say, i ended up getting a part time job there.
Harking back to your question, theres no prerequisite subjects needed for a job at a lfs, however, a basic understanding in biology and chemistry might be useful, just to know how things work and why. You might know a fish needs an ideal pH of 6.8 say, but knowing what exactly a pH is might help you answer some of the tricker questions you get, as well as deepen your own understanding of the hobby. From my experience alone, 99% of people who visit an aquarium have no clue about anything fish related, so patience is another big plus! As you'll no doubt have to explain basic concepts like water changes over and over and over and over again.