I am *speculating* that the single
Puntius denisonii is way too lonely, and therefore behaving erratically. As with schooling fishes, they do a lot better in groups, and tend to "behave" better too (think Tiger barbs). So it may be worthwhile considering getting more of them, 5 or 6 at least.
On the other hand, it seems as though it may not be eating enough, which is why it is picking up food from the substrate. And barbs do appreciate some amount of plant matters in their diet. So perhaps, more frequent feedings (in small amounts), inclusive of some green stuff will distract them from your plants.
Disclaimer: Mileage may vary.

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