I'm new to fish, and this is my first tank. I have a 10 gallon tank with heater, filter, airpump, gravel, and some plastic plants. Today is the 3rd day I have had the tank. I have two guppies, an x-ray tetra, and a rainbow shark. Today, I've noticed that the rainbow shark hides perfectly still under a plastic plant any time the light is on. When I shut the light off, it immediately swims around freely. Is this typical behavior of a rainbow shark? And if not, what would you recommend to allow it to have more freedom in the tank?