to AA!
I get the feeling from reading your intro that your LFS has not fully told you all that will be required of you to establish a healthy happy aquarium.
The first thing I recommend is you read the article in the link below on cycling your tank with fish. Not sure what your LFS told you, but cycling means establishing the crucial bacteria in your tank that help your fish from poisoning themselves.
I just learned about cycling but I already have fish. What now?!
If at all possible and return the zebra danio's ... you can do a fish-less cycling (link below) which requires a little less work on your part. The LFS suggested Zebra Danio's because they are hardy fish that many use in cycling their tanks.
The (almost) Complete Guide and FAQ to Fishless Cycling
The next thing I recommend is getting a quality liquid test kit like the API test master. Strips are not reliable. You need to monitor daily ammonia, nitrites, nitrates, water temperature and pH. You'll also need a quality de-chlorinator like Seachum Prime ... if available in the UK.
Could you also tell us the size of the tank and the filter you are running. That info is crucial in helping keep the ammo, nitrite and nitrates in order.
I'm certain .. by this point your wondering what you've gotten yourself into ... this is much more complex than the LFS made it sound. In truth it is a bit more complex than LFS's say .. but not necessarily harder. It just requires some due diligence on your part. The reward is a well established tank saving your time, heartache and $$$$$$$$ in the long run.
Feel free to ask any other questions. Members at AA are interested in helping members have happy healthy aquariums which ultimately means less stress for you.