The nibbling is likely one of the barbs, and of the two (Rosy and Denison) most likely the Denison. If you sit very still in front of the tank for an extended time, an hour or more, you may see it and that would pin the culprits down. Many barbs will pick at some plants, sometimes to graze algae and other food, sometimes the plant themselves.
Denisons are prone to nibble on soft-leaf plants. Their natural diet includes algae, so vegetable matter like blanched spinach, zucchini and cucumber have been said to help curb their appetite for the plants. I've no idea how voracious they can be nibbling plants, just that they tend to do so.
Byron.