You my want to check with your water provider to see if they use Chloramine, or straight Chlorine as a disinfectant.
If they use Chloramine, this will often give a slight Ammonia reading, right after water change,
and realize, depending on how well stocked, or overstocked your tank is,(as many Rift Lake tanks tend be, .....fish are constantly putting out ammonia thru the gills.
If you are overstocked, and your shy on bio-media, it may not be able to keep up with ammonia output so you will get slight ammonia readings. How much filtration are you using?
And are you regularly vacuuming substrate? or over feeding? any left over food, or detritus stuck in or under rock work will be rotting, putting out ammonia.