I've just added my first goldfish to my coldwater tank. I just have a question about testing ammonia levels. To make sure the water is ok etc I have bought a couple of test kits. Both are strip tests (I know some say liquid kits are better but there wasn't one in the store). I have tested the ammonia level today, the kit says to dip the stick and then read the colour after 60 seconds. I did this and it matched the colour for 0ppm. I just wanted to ask if the strip will continue to 'develop' over time. I left it on the side and after about 20-30 mins I came back and it had changed colour, and I am hoping this is just normal and not a true reflection of ammonia levels.

THanks for any advice