Tell them what you told us. That you love tropical fish, its your hobby and you need some money to keep it going. Tell em you will mop floors, take out the trash, service the tanks, whatever. The worst they can do is say no. It was a long time ago when I was 13 but I think I left my name and number.
Tell them what school you go to. Just be enthused. Don't be afraid of someone saying no. Thats part of growing up. BTW the fish store I worked for was a mom and pop. Owned by 2 brothers really. It was called Sunset Aquarium in/near Hollywood. Brings back memories.