Welcome to MFK!!!
Alot of ammonia removing/fixing additives will cause a false positive test result on several litmus paper test kits and reagent liquid test kits.
I suggest testing your tap water first to be sure you're not getting the ammonia from the municipal supply. If not, either run resin to remove the bulk of the ammonia removing additives or perform several additional water changes over the next week (without using any additive that deals with ammonia - just use plain dechlor).
And, as stated above, provide us with some basic info like:
tank size
fish numbers, species, sizes
filtration
water parameters including temp, pH, nitrates, etc.
maintenance regimen
feeds used
and any other info/pics. The more information provided, the more useful and quick the recommendations will be.