I don't know that their behavior day one in the tank is great barometer for how favorable or not favorable conditions are. Your original post says parameters are good, but only lists nitrate, nitrite, and ammonia (the latter 2 are not really relevant for tanks that are already cycled). Phosphate? Alkalinity? pH? Not sure if those will affect a feather duster, but certainty could. Also tank is looking very new, so unless you are specifically feeding them/broadcasting tiny foods they are likely not getting food.
All I am saying is that there are a number of factors at play and we don't have all the details.
All I am saying is that there are a number of factors at play and we don't have all the details.