It will not kill of the 'good' bacteria needed for the continuation of the nitrogen cycle. Bacteria are not susceptible to the osmotic shock of hyposalinity. Unfortunately, ,many other microscopic/macroscopic critters are. If you use hyposalinity and have live rock and/or sand present you will get some die off and a small (hopefully not a large) ammonia release from the die off. This is why many people do not hypo their DTs and resort to use of a QT. In the QT people may put a small amount of LS or LR to help establish a biologic cycle.