If you want to kill your corals you do!!! Never use straight ro water as a second wash off dip.....Even as a single dip it is very very stressful to acros..
upon returning a coral into the tank after a dip I suggest putting it thru a couple stages of seperate vessels of tank water before the return...This will let it purge some of the active ingredient from it's slime coat..(if crabs in the tank pick at the slime coat once returned without doing this it can poison them!!!)