how about this:
Water element (the water in the aquarium)
Wood Element (the plants in the aquarium)
Metal element (use some white and gray rocks in the fish tank)
Earth element (the rocks and gravel at the bottom of the fish tank)
Fire element (gold /yellow, orange and red colors of the fish, as well as the aquarium light)
Water element- Duh, water. lol
Wood Element- Stand, canopy, etc.
Metal element- Power supplied by wires.
Earth element- Substrate.
Fire element- Heater.