The water change is a good idea...there is little downside in that anyway.
No matter how carefully you transported or obtained that LR
, there is going to be some stuff that dies in the process. So some spike is very normal.
Do you have additional bio-filtration beyond the 25lbs of LR
that you already had? If so, make sure you DON'T clean the filter or anything for the next short while. As you are adding LR
to an existing system, the capability to handle the new ammonia is dependent on that existing 25lbs of LR
and any other filtration you had established.
An established tank should be able to adapt quickly enough that you're not likely to lose fish. More sensitive life might have a problem.