I respectfully disagree with NM that it will "kill" your LR and LS,
It will reduce the bacteria and will greatly reduce if not kill off most of the infauna (worms, pods etc.) in your tank, but some do seem to survive.
But it will not completely kill everything off. I had to do it in a 210 because I had no other options, there were no crabs or snails in the tank it was a 210 FOWLR, It is certainly a last ditch effort and if you can remove the fish and do hypo in a seperate tank that is certainly prefered, but if you do not have choice, you can do it in the main display. You may need to add some cultures of LS back into your tank to re-introduce the infauna, but the bacteria will mutiply rather quickly as the salinity comes back up.