The problem may be with what your tapwater is treated with. If it is treated with chlorine only, and tests of tapwater show no ammonia, I would surmise tapwater is not the problem.
However, many municipalities now treat their water with chloramine, which is a compound of chlorine and ammonia. Chloramine lasts longer as a disinfectant of tapwater, killing bacteria that can cause illnesses, than chlorine. Chlorine has a tendancy to evaporate quickly, resulting in it being less effective at killing bacteria over the long term.
Also, some test kits will not show ammonia present if chloramine is used in tapwater. I use an Aquarium Pharmaceuticals ammonia kit that shows we have about 1 ppm ammonia in our tapwater because of the chloramine treatment.
For many years, we've used a simple 50 gpd RODI unit, but recently tested the water coming out and it contains about .25 ppm ammonia. After finding this out, and asking around a bit, we've upgraded our RODI unit to include two granulated carbon filters.
The upgraded unit is configured for the water to go into a 1 micron sediment filter, an extruded carbon filter (both of which are included in RO and RODI units), two granulated carbon filters, the RO membrane, then, finally, the DI resin bed. We just got the upgrade over the weekend and are still getting it fine tuned. Haven't had a chance to test for ammonia yet, but the water coming out of the new unit smells better already.
Anyway, what you need to do is contact your local water provider and find out if chlorine or chloramine is used. Talk to the people in the lab at the treatment plant for the most accurate answer. Most people at the water department here, except for the lab folks, always tell us that our water is treated with chlorine. Dumb people, but that's the way it is
