From what you are telling me, the filter was not the problem.
Your bio filtering capacity adjusts to the load you have, which in your case was pretty light. These fish are a pretty big jump. It looks like you doubled the biomass or more.
This problem should correct itself before too long as the filter systems comes up to the new demands. You should do good size water changes as often as required until this occurs. In addition to that you may want to use a product that temporarily detoxifies the ammonia. There are many.
I would wait to have a go at your filter until this problem settles out. And then be careful to avoid too much disturbance to the filter media when you do clean it.
Edited to add: This all assumes that your filter was properly sized for 90 gal to begin with.