Hehe, you've kinda named situations for three different lenses.
Close-ups
Candids
Landscape
For close-ups you don't always need reach, you need close focusing, which is what macro lenses are designed to do. They also make great portrait lenses because they're so sharp. Usually for aquaria you don't need more reach than about 100mm because our tanks are typically only a couple to three feet deep, but if you plan on photographing something deeper think about 180mm.
For true candids you want something with reach, at least 150 - 180mm IMO, so you are far enough away from your subject to observe them without them observing you.
For landscape shots with family/other people in them you want something wider to capture the scene behind the people, maybe an absolute max of about 50mm obviously depending on the scene (sometimes even 20mm still isn't wide enough).
Considering everything above, if you only have the money right now for one lens, something you might want to think about is keeping the kit lens for the landscape stuff and stopping down the aperture to the sharpest point (probably around f/11 or so), and making your purchase a 70-200mm and a 500D attachment. You can use the 500D on either the kit or the new lens for close-ups, and the reach of 200mm is perfect for candids and a good start to getting long enough for wildlife shots. I have seen some amazing shots with that combo.
Hope that helps some!
