I have been using bottled water (very expensive) for my fish. I decided to try and use some tap water, experimentally.
I took it and ran it through the brita filter, then set it out for 24 hours to age. I put it in my fish tank (100% change) along with some API Tap Water conditioner.
Next day I measured the ammonia with a strip and found it was showing ammonia, though still in the "safe" range. This is in a very small 1/2 gallon tank.
My other fish, who are in bottled water, showed NO ammonia on the test strips.
So I decided to do another 100% change for this tank and add some Prime and see if it does better. We'll see.
I tested the aged water by itself and it too showed ammonia, though still in a "safe" range. What this means is that the tap water is breaking down to ammonia even with NO fish in it.
This really sucks. I won't drink the tap water here for years now and it only proves I was right not to. This is an annoying problem.