API Test Kit Question

[DCtank]

For weeks I've been having trouble getting my ammonia readings to zero. The color in the test tube isn't nearly as green as .25ppm but it's not as yellow as 0ppm, either. Water changes yielded no results. I've been using bottled spring water because my tap has high ammonia levels so I decided to test the bottled water. It came out the exact same color as my tank water.

So, my questions:

Is it possible my tank's ammonia is 0 but the yellow on the card simply doesn't match the tube perfectly?

OR

Is it more likely the bottled water has a low ammonia content?

I emailed API with these same questions but got a confusing response.

 

[Rivercats]

Sounds like the bottled water might have a very low ammonia content. Is your tank planted? And is it fully cycled? When I've tested my tanks with the API ammonia test I've always gotten the same yellow as on the card for 0. It sounds like your getting a barely readable amount. But I'm not sure I'd be too worried if your getting some type of yellow reading. There can't be very much detectible ammonia in that water.