I used to have a problem with my tap water. Nitrate, nitrites and ammonia were 0ppm but 1 week after a water change my nitrate were 140+. The filters were ok and managed to cope with converting the ammonia and nitrite to less toxic nitrate but I could never get the nitrates to drop. After 2 big water changes the nitrate would drop to around 50ppm but over the next week it would rises steadily back to over 140ppm.
In the end I had the water tested and contacted the local council. I was advised the well in my area came up through an old forest. While the water looked good on test kits it was loaded with dissolved organic mater that broke down in the tank. This was confirmed by the local lab (very expensive test as well!!). So, there was the reason for the high nitrates...
Maybe there's something non-toxic and organic in the water tanks creating the issue.
If you can't get the ammonia level down doing water changes, you'll need to find the source. Maybe it's the water you're using for water changes. Maybe it's something else in the tank breaking down - a rotting log, detritus in the substrate etc. Logs can rot from the inside out if they have a soft center. Do you use UGF? If so, when was it last cleaned well?