You would have to densely plant the tank for it to use up ALL the ammonia. In fact there's something known as a silent cycle, which happens in densely planted tanks sometimes.
The BB, once established, don't care where the ammonia comes from and it will be utilized by them just like fish waste is. So long as you condition the water with a product like Prime that neutralizes chloramines, it should be ok.
I need to check on some questions about the water chemistry.. I'm no expert on it for sure, but I keep learning more as I go. Let me go check, something is niggling at the back of my brain.
Edit.
Here are some links to articles on water chemistry, particularly regarding ammonia and ammonium. NH3 and NH4. Ammonium, NH4, is not harmful to fish, it is chemically bound up. It can be used by the BB, but won't hurt fish.
NH3 is free ammonia, and is the one that harms fish. Temperatures, hardness and pH all affect how much NH3 and NH4 there is in realtionship to those factors. Cooler water has less ammonia and more ammonium, but it's dependent also on how hard the water is and how alkaline it is. Your water is fairly hard and the pH is high, so that's good thing for you.
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Prime, and other products like it, don't actually 'neutralize' ammonia. They break the bond between the chlorine and ammonia. This can leave the ammonia free, as NH3, which can cause a problem for the fish. Prime does bind ammonia for about 24 hours as ammonium, which should be long enough for a cycled tank to convert it.
But most tests, API included, do not distinguish between NH3 and NH4.. the reagent raises the pH of the water sample to over 12 and the reading you get in ppm is a combination of both types, aka, Total ammonia. So you can get a reading of ammonia that is in fact false and not harmful to fish.
Seachem makes a test that tells you how much is free ammonia and how much is ammonium and I think in your case you really need the Seachem test, it's the only way you can find out how much of that ammonia reading you are seeing is actually NH3 and how much might be NH4.
First link is to Seachem's site, their explanation of how Prime works. The last link's article is a bit technical, but it's very good and it might help you a lot.
One suggestion for tap water with high ammonia is to age it in a large container. This is what fish keepers used to do thirty and more years ago when most municipal water only had chlorine in it. It gases out fairly fast, so aging it was all we had to do.
Wide surface are helps, so maybe a trash container would work. You need a cycled filter on this container. Treat the water with a simple dechlorinator first.. not Prime! Something with just plain sodium thiosulfate will dechlorinate it, or you could allow it to gas off by itself and then put the filter on it. The chlorine
can harm the BB, so you can't have the filter on water that still has chlorine in it.
Then you allow the filter to convert the ammonia before using the water for fish. This may solve your issue if in fact your ammonia readings are not false.
http://www.seachem.com/support/FAQs/Prime.html
http://www.cs.duke.edu/~narten/faq/water-treatment.html
http://www.extension.iastate.edu/CropNews/2008/0421JohnSawyer.htm