For those of you thinking about adding fish to that "lake" in your layout, just remember they can end up being as much work as cleaning the tracks. And you can't overfilter the water. Its always easier to maintain less fish with more filtration. Over the last few decades I've kept everything from piranhas, to lion fish, to moray eels, to sharks, to seahorse. When we go to a public aquarium, I like to point out the exotic fish and say, "I've killed one of those".
Of course, you can always buy some cheap feeder goldfish and let them fend for themselves. That's what my mother in law does every spring.
Ben, it sounds like your's is just deep enough at 2'. "By the books" you would want a 4' deep section for really cold winters. I built mine that way originally on one end. Unfortunately I live in a forest of mature oak trees. Over the years I gave up trying to keep them cleaned out. Now it's more like 2' deep. And oak leaves are too acidic for fish. However, in the summer I do have to worry about giant mutant frogs sitting on the tracks, right in the middle of the bridge. ;-)