Let me pitch in my 2 cents, double-check your tap water just in case, quality can change day to day. Your city may be adding chloramines, which release ammonia when you use a dechlorinating agents. Test your tap 15 min after adding the dechlorinator, to see if that could be the source. If it is, no amount of water changes would help, only a well established bio filter after adding prime or other ammo detoxifier; should be gone in about 24 hrs.
Also, I read you rinse your filter in tap water, this will kill your bacteria as well. You should only rinse filters in water coming out of your tank during water changes.
Good luck, keep us posted.