Oh, thank you, Candice. :)
I use acrylic Pet Keepers from Petco, both for them to live and as photo tanks. For the fussier fish, you sometimes need to get them situated in the photo tank hours before the shoot. :)
I would love to take them outside for photos (because, yes, I do use natural sunlight), but my fish room is upstairs so that I can more easily control the temperature. Soooo, I have a small table in front of a window, and that's where I set up. :)
As far as getting these poses, I don't do anything special. I let my fish swim around and do what they do, and I snap pictures. :)
Most of them, to be honest, are doing their "FEED ME" dance. :P They are rewarded well after I've taken the photos, of course!