pictures and video of the fish?
what are you feeding the rainbows?
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If you have 2 males, they will display to each other and that is normal behaviour. If you have a male and female, the male will display to the female and they will usually breed among the plants.
If you only have 2 rainbowfish, they are probably bored and that could be why they are picking at the bristlenose. Rainbowfish naturally occur in groups of 100+ individuals and need to be kept in groups of at least 6 (preferably 10) or more. You can mix different species as long as they are all the same size and they will hang out together.
They might also be hungry. Rainbowfish need lots of plant matter in their diet and should be fed regularly, especially if they are young and still growing.
Make sure you have driftwood and algae in the tank for the birstlenose.
Put some pvc pipe in the tank for the bristlenose to hide in. Use 1-2 inch diameter pvc pipe that is about 6-8 inches long.
Have lots of plants in the tank so the rainbows don't see the bristlenose and they will be less likely to harass it.