My tank almost crashed when I added 50 lbs of LR all @ once.
this guy in our area was breaking down his system and selling everything, so w got a great deal on his LR ($1/lb :jumping: :jumping: :joy: ). Our tank was 4 mo old, already cycled and coming along great. We brought the rock home and aquascaped (it looks great), a week later we wanted a new fishy so we went to the lfs and our amonia had sky rocketed. We did 4 25% water changes each one day apart, that took care of the amonia and nitrites. It took about a month to get the nitrates under control. The good new we did not have 1 casualty.....very lucky I know!!!!
I've also heard if you add too many fish at once, the tank cannot adjust to the new bioload and it crashes..... Hope this helps!!!