Valchitsa

"Sharky", my rainbow shark, has decided to move away from his cave and moved into a... java fern. He normally would be in his cave but for two days already he has left it and just hung out on the fern... head down. He just swims backwards up the fern until his tail is resting on one of the leaves and then he just hangs there head down. From time to time he will leave and swim through the cave, but then he just comes back and hangs himself upside down again. I've seen him do that before with the heater, but that was because he was a little chilled and I did turn it up a bit for him. So what is up with the fern? Oh, and he almost never goes to the other side of the tank, even though there is another cave there. He ventures out there at night sometimes, but still rarely. Why such dislike of the other cave? Why the sudden fern-hugging behavior?

TFA101

he's established a territory and he's defending it. I know a family who had a flying fox, a close relative of your shark, who would just veg in a piece of driftwood head down, and hover there unless the food was being given or he had to fend off an intruder. This is just your shark establishing a new territory, maybe taking it a bit too seriously.