How high is your ammonia reading on your tap water?
If you do have ammonia in your tap water and your water changes are single very large ones as opposed to frequent small ones then you need to be using prime to render the ammonia safe until your BB can convert it. If it's a well established tank the ammonia should get converted pretty quick.
If you don't have ammonia in your tap water then YOU are doing something, or there's something going on in your tank that's not right. If you are overfeeding for instance and you have decaying food.
But like I said, the first port of call is finding out what your ppm out of your tap is.