5 months old should be ok. If it was older with black layers do not use it has toxic layers of broken down food and detris that will make the tank inhospitable. If your is clean (which it should be) you will be ok
The only problem you will encounter is when you take the sand out it will cloud the 65 and when its in the 155 it will cloud. So fish will be in buckets for a few hours (suggest hospital tank for that duration)
BTW what livestock do you have?