Have you used anything in the tank like ammo lock or anything similar that is supposed to detoxify ammonia? If you have the altered ammonia is still picked up by your test kit as ammonia even though it's supposedly inert. I have this problem with one of my tanks atm - ammonia reading is off the chart, real reading of harmful ammonia? NFI. Only way past this is lots of water changes and will be a while until I can be sure my test kit is reading true again