Piranha Tank was developed expressly for the purpose of pursuing Fish Tank.
His mind is driven by the need to feed. And what Piranha Tank likes to eat is... Fish Tank!
I love these NiZn rechargeable batteries. They kick butt for a rechargeable AA, putting out as much as 1.8V when fully charged.
Still in development, but he can drive around, avoid obstacles with his IR sensor and two front-mounted bump switches, swish his tail and wiggle as he "swims", and search for the brightest source of IR light.