API Stresscoat & water conditioner does nothing to neuteralize ammonia and will actually raise the ammo level if you have chloramines in the water. Check out
http
/www.thetropicaltank.co.uk/rev-cond.htm to find a water conditioner that will help with ammonia. Some tests will still test positive for ammonia as they detect ammonium the same as ammonia. Ammo-Lock and other products work by converting ammonia to ammonium, which is harmless to fish but still used the same as ammonia by your bio filtration. A two reagent test will not give a false reading with ammonium.
Rehoming some of the fish would be best, as has already been stated. You should be changing water a lot more often than once monthly with that stocking, 50% 2 or 3 times weekly is needed.