As the days pass the creature grows sluggish and unresponsive. It lies listlessly at the back of its tank. Its inner light dims, flickers, and is gone. If you are hoping that it will recover and resume its usual activity, the hope is a vain one. It all happens quite quickly. Not an hour after it goes dark and dead, its body begins to bloat. A thick viscous material coats its swelling flesh. This goop seems almost to glow, but it's probably just decay. Probably just a little of that strange phosphorescence leaking out of Nova's sad remains.
It seems like a good idea to take poor Nova out of the tank. If you don't, the stick shapeless mass that was your pet will bob to the surface of the water. Either way, once it hits dry air the ball will dessicate swiftly. It crumbles to nothing in a matter of hours.
If you should wake in the middle of the night, you will see something quite strange. The sad little heap of dust has ... puffed up into the air. Reformed, somehow. It is a nebulous little ball of darkness. As you watch, you see it solidify bit by bit. Drawing itself together. The darkness at its core will grow thicker and and heavier as the days pass, though it will retain its corona of swirling light.
Your dreams will solidify, too, until they consume your waking life. Though the other place is the same age as your Earth, there is a feeling about the life and culture here that is unmistakably ancient. Intelligence rose to primacy aeons ago. This alter-world hit its peak hundreds of thousands of years ago, and is now settling. Stagnating. Despite its beauty, the world is bland and slow. It is a garden, and like a garden, it is utterly tamed. But change is coming, change. You, or the alter-you, can feel it.