The kit lens will give you an idea of what focal lengths you need for going with a prime. A 30mm or 35mm will go well with your crop camera.
Also look into speedlite flashes. They will help a lot with kit lenses getting enough light into your tank. Yongnuo speedlites are inexpensive and very good. Cowboy Studio triggers are also inexpensive and very good.
Any lens with a maximum aperture of f/2.8 or larger is considered a fast lens. A maximum aperture of f/5.0 or smaller is generally considered a slow lens.