How much water is that? You could use any number of small schooling rasboras, with some freshwater gobies and killifish. It depends on what you enjoy though, a few large bright fish, or a large number of schooling fish. I'm not sure if cichlids would be a good choice, as I'm pretty sure that they need hard water, and that might be bad for the plants/frogs, especially with many of them coming from south america. Just something to keep in mind, but some rams might work, that's a fish I've always wanted haha.
Anyways, this is a spectacular tank, and I can't wait to see it finished. That pond, stream, and multiple levels are really an amazing idea. Good luck with it all!
Edit: you might want to stay away from duckweed, it takes over incredibly fast and is difficult to get rid of, especially in high light, low surface current situations. There are many other floating plants that you can use, that are bigger, grow slower, and filter the water just as well, such as water lettuce.
Last edited by Ckent42; 05-02-2012 at 12:17 AM.