Calibrate your test kit. By my calculations, 1 drop of a 10% ammonia solution (which is what you get from ACE) in 1 gallon of water should yield about 1ppm. Regardless of the reading you get for 1 drop in gallon, I'd suggest taking 1 cup of this solution and mixing it in turn with 1 gallon of water. This should give you .0625 ppm. You should be able to detect this (just barely). You'll need to compare side-by-side with a test containing no ammonia.
Are still detecting ammonia? If so, do water changes, add plants as Hoppy suggests, or use Ammo chips. Also reduce food. Do this until the reading goes back to 0. However, all of these will slow down the cycling process. I hear you can also use Amquel, wish supposedly neutralizes the harmful affects of ammonia while still leaving it available to the nitrogen eating bacteria (I'm not 100% sure of this myself).
One last suggestion is borrow anything (gravel, filter material, ornament) from a well cycled tank and put it in yours (in the filter if possible). Chances are that alone will handle the ammonia waste produced by your one danio.
I've done fishless cycle fairly quickly. Using ammonia and seeding with some mulm, I've cycled a number of tanks (consuming 5ppm of ammonia in less than 24 hours) in 10 to 21 days. If you're just cycling for a lightly stocked tank, you can get away with far less than the 5ppm rule before introducing fish. With 1 Dania in 20 gallons, I'd stop cycling once any indication of ammonia consumption was visible. Start with 2-5ppm of ammonia. Once you see a dip, replace all your water and put the Danio in. [Yes, I know, the nitrite eating bacteria will trail the ammonia eating bacteria, but once the ammonia eating bacteria are established enough to measure their consumption, the nitrite eating bacteria should be far enough alone to handle to waste from one Danio. But, if you want to be sure, add about .25ppm of ammonia and check that there is no ammonia or nitrite 24 hours later.]