I think you have an inherent conflict of interest in this scenario. Since NPS corals don't naturally host clownfish, any contact with the fish will most likely irritate the polyps and cause them to stay closed. This is a bad thing for a coral that needs (in many cases) to feed continuously. Additionally, if the NPS has a stony skeleton, the clownfish's movements may cause the skeleton to irritate and/or tear the coral's tissues, which can lead to infection and death for the coral.