Looks like you severely knocked back your Amm-nitrite bacteria.. there little dead bodies created a heterotrophic bacterial bloom..
fortunately I'll assume your pH was around 7 so most of the ammonia was not toxic..
this doesn't apply to nitrite btw.. toxic at any pH..
any chance you were feeding CO2 into your filter???
Btw. nitrogen conversion occurs everywhere in the tank..
"A man with a watch knows what time it is. A man with two watches is never sure"