I bought an arrow crab because it was one of the coolest things i had seen when i was diving but then i did a lot of research on them and they can eat/bother a lot of things from snails, hermits, bristle worms, feather duster, brittle stars, small fish, ect. but they also said that arrow crabs are very much individuals and some won't do anything and some will be destructive. Mine so far has been an angle and he is becoming rather big. they said that when they are bigger they are more destructive. The only thing i have seen mine eat other then the frozen squid i feed him every other day is one dying/dead snail that i think my blue leg hermit crabs killed for his shell. at the moment i only have one piece of coral in my tank but it is still alive and healthy and I have tons of Christmas tree worms and feather duster and they are still fine. i have anemones, hermit crabs, other assorted crabs, brittle stars and other assorted things and everything seems to not be bothered by him. So i guess i would just keep an eye on him and keep him pretty well fed and he probably won’t touch anything.
i don't know much about queen conchs but i do know that a lot of the conchs that exists are carnivores i had not in my tank but in a tank at work a crown conch, olive snail, Florida fighting conch, and a lighting welk and they would go crazy when i would put frozen squid in the tank. The lightening welk and olive snail who u usually don't seen cuz they cover themselves would pop right up and move faster then i though a snail should go. so i guess i would place more money on the conch if it is a carnivore. i
HTH 
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