By changing the filter media last week, you may well have created this problem. The bacteria that process ammonia and nitrites live in the filter media. Instead of changing it you really want to just rinse it out in used tank water and put it right back into the filter. You really want to just get the solid looking deposits out of and off the filter, not deep clean it. That way the bacteria that are growing in a film on your media can keep right on working.
Thanks for the advice.

