I love my Nikon 50mm 1.8. For $100...I don't think there is a better lens value out there. You can do general aquarium photography with it, only problem is it's a bit long in focal length to do full tank shots unless you have a small tank or large room. It's an awesome lens though for the kids and as a general walk around lens. The 30mm focal length is great too. For landscapes, I love my Tokina 12-24mm but you are looking at a $400 lens. Cheap as far as lenses go, but not cheap as far as my wallet goes...lol Check out keh.com for used gear at great prices.
Camera Pimp member #1