My grandfather has both of my dream tanks. One is a 500 gallon marine tank that I fell in love with when I was a kid, and begged my parents to let me get a fish tank of my own, and why I bought a tank as soon as I got my own house (even though it annihilated my video game and beer budget for several months).
The other is in his sun room. The room was built over the concrete slab that was originally a patio, and he cut through the slab and built a koi pond right in there. It's beautiful, rockwork, plants and all, and with a lot of messy trees and poor rain quality, infinitely easier to maintain than an outdoor pond. I love the idea, and in theory I could do that here (My den was built over a patio the same way), but my cat would almost certainly view anything like that as a free dinner buffet. Outdoor ponds are out of the question due to the number of stray cats in the area.