Of the three options you mention, the chaetomorpha is the best. Less maintenance, no feeding requirements, tolerant of almost all environments, accepts a wide variety of lighting including NO
and can occassionally be used as a food source.
Xeniids will require better water quality, more maintenance and increased cost from lighting. They are also much less tolerant of nitrates than you may have been led
Scallops are strict filter feeders so while they may process some/selected nutrient within the system, you will need to suppliment their feeding which detracts from the overall scheme of "reduction". They are very picky if you will about the particle size of their foods (species depending) and will often starve over the long term. Even in a nutrient rich system. Keep in mind they are not a means of export rather they process nutrient into something else. While this option may sound the most intrigueing, it would also be the most work and highest rate of failure. They also have short life span, 2 years at most depending on age at harvest.