Cycling - No Nitrates?

Hi all, I've been using Think Fish for a while now but this is my first forum post!

I've recently upgraded from a small tank to a 180 litre with my original tank having been running for about 18 months and containing just Danio's. My new tank has been up and running with fish for just over 6 weeks but my tests have never shown any Nitrites, which has me confused. Can anyone shed any light on this??

My initial readings were: ph 7, Ammonia 0.5ppm, Nitrites 0ppm, Nitrates 80ppm. The ammonia dropped fairly quickly to 0.25 and then to 0 in just over a week (daily tests). Nitrates dropped a bit more steady but are now around 20ppm. My ph is now around 6.6 - 6.8. N.B. As well as the Nitrites no show, the lack of any significant ammonia spike also seems unusual? For info, I'm using the API Master Test Kit.

Bit of background: I added only used (although thoroughly washed) gravel to my new tank and ran it for over a week before adding any fish (the first fish added were the Danio's from my existing tank and no more fish were added for another couple of weeks). Also, again before adding any fish, I added water from my existing tank daily, put a piece of my existing (and un-cleaned) filter media into the new filter and added some fake plants which were again from the existing tank.

When my Danio's were transferred to my new tank I had no use for the original tank so added the rest of the water and also transferred the filter - this has been running in my new tank up until last week.

Would the above steps have been sufficient to generate the ammonia and nitrite readings I've had? For info - I spotted another post which said that nitrite readings can sometimes shows a nil because they're 'off the chart' so I have re-tested my water for nitrites with a 1 in 10 dilution; again the reading was nil.

Any help/advice would be appreciated!

Thanks Sue