I had something similar happen but I think it's more the water coming from the tap here has ammonia/ammonium in it as well. Started getting ammonia readings in the shrimp tank daily... only .25 - .50 but too much anyway. Tested water from my well established 125gallon to make sure it wasn't a faulty kit and that reads 0. Tested the water from the tap and got a .25 reading. I've been using Seachem Prime now and although we still get a .25 reading daily from the tank the shrimp are doing fine. I do add about a 1/4 ml a day of Prime straight to the tank, which may or may not be a good thing, but the shrimp are not doing bad even though the ammonia tests are positive for ammonia. If you can find Prime at any of your fish supply stores I would try it.
From Seachem website:
"Prime™ is the complete and concentrated conditioner for both fresh and salt water. Prime™ removes chlorine, chloramine and ammonia. Prime™ converts ammonia into a safe, non-toxic form that is readily removed by the tank’s biofilter. Prime™ may be used during tank cycling to alleviate ammonia/nitrite toxicity. Prime™ detoxifies nitrite and nitrate, allowing the biofilter to more efficiently remove them. It will also detoxify any heavy metals found in the tap water at typical concentration levels.Prime™ also promotes the production and regeneration of the natural slime coat. Prime™ is non-acidic and will not impact pH. Prime™ will not overactivate skimmers. Use at start-up and whenever adding or replacing water"