Do you have all the results for ammonia, nitrites and nitrates written down? If so, can you post the results here please.
Having a low to medium reading of nitrates in tank water is not harmful in itself - so I can't understand why they would refuse you fish for this reason alone. Was there an ammonia reading? Or nitrites? Those would be harmful to fish in even the smallest level.
And when they tested your water did they do it with a strip test? Or did they use a liquid test kit (i.e. put a sample of your water in a test tube, added drops from a bottle, shake it, add more drops, shake again, etc). Did you see how they did it?
Also, how long has your tank been set up and did you do a proper fishless cycle on it? Or have you just put water in it, turned on the pump and left it 2 weeks..?
The more info you supply the better help we can give.
Regards - Athena