In a tank like that, low to the ground and with no real cover and an open layout, the fish will probably stay shy IMO. The lower fish are to the ground, the shorter the stand is, the more shy they tend to be, in my experience. I believe it has something to do with their instincts to avoid bird predators, and in a wild fish these instincts will be strong. Also the cover- I would add lots of cover in the upper reaches of the aquarium. Floating plants work well, as do large branchy pieces of driftwood. Right now, the fish is going to stay in the cave, since it seems like the only safe place. If it swims out into the open it's totally open to predators. It's also the only sizable fish in the tank so feels like a target.
I'd try the decor first. If that doesn't work, try putting it on a stand higher up. Keep the barbs in as dithers. If none of that works, might be best to just put it in the pond now.