How do you perform your water changes? Is it straight from the tap or aged? Do you have city water or are you on a well? Have you tested the water coming out of the tap for ammonia?
My gut instinct tells me that an ammonia spike immediately after a water change means you have ammonia in your water supply. This isn't a huge deal because we have products like Seachem Prime that will neutralize that ammonia and make it harmless to the fish until the BB can take care of it. I haven't used Fritz Complete personally, but from looking at the datasheets and such for it online, it looks like it has a similar or near identical chemistry as Seachem Prime (it looks like they use some form of hydrosulfite).