[Since you have only two you can do it either way IMO. Treating the entire tank is better, but you can always go back and do that if you see them return. Personally I would treat the entire tank before you start adding colonies with crabs in them, but that is me. It only takes one loose one to repopulate. How is that for a politician style answer. 
I have pre-treated a couple of times. I guess it was successful, but I never knew for sure if their were any bugs on those SPS to begin with. It is too easy not to do, even if you don't suspect there are any.]
I have pre-treated a couple of times. I guess it was successful, but I never knew for sure if their were any bugs on those SPS to begin with. It is too easy not to do, even if you don't suspect there are any.]