How do these crabs actually hurt our systems?
Many crabs are quite reefsafe - but as they get larger they can be clumsy and knock our rock work over - and are only algae grazers.
Some crabs start off ok, but then become opportunistic and take snails/hermits etc
And there are some species of crab which are just out and out carnivores and will go for small fish, or even larger sleeping fish.
But we do need to remember, they didn't ask to end up in our tanks, they went to sleep in their hidey hole one night and woke up in a glass box mant thousands of miles from where they started, so remember we also owe them a duty of care, sump them of return them to the shop, but don't just kill them because they have eaten something that cost you money.