Building the app was very straight forward. In typical Unity style, I was mainly dragging and dropping prefabs. Google's documentation is up to date and very helpful.
Because the Tango is a bit more aware of its environment than a regular smartphone the fish feel more planted in the environment. You move the device from side to side and back and forth and the fish still appear in sync with the camera feed. It's pretty cool.
There are lots of things that could be improved, like matching the lighting on the fish to the information from the camera as well as occlusion mapping.
Originally I was going to put the Tango in a VR headset so this can be experienced that way, but I think for the meetup a handheld experience will be more fun and social.
Hmmm... I wonder what the market for augmented fish tanks is like? That Pokeman thing sure seems popular :-0