Slider is right. Adding your tank water gradually (and by little amounts at a time) to the bag with fish is a very good way to acclimatize your new fish. Also, maybe try buying 1-2 zebra danios at a time. If you add all 6 of them at once to an 8 gallon tank, your nitrite (and ammonia at worst) could rise very quickly and kill your fish. But when you stock your tank gradually, then there is enough time for bacteria to build up and support all of your fish's wastes. You could be adding 1-2 danios every week.
Besides, just a personal thought. I think an 8g. is too small for zebra danios. This is not to discourage you - it is totally up to you what you'll do. But from what I know, they're really fast swimmers and they need a lot of space to accommodate their temperament. I would say, at least a 20g. is good for fish like zebra danios. It is true that they're great beginner fish (as are white clouds) but I believe their temperament should also be considered when buying them. You wouldn't want to be trapped in one small room your entire life, would you? Please don't treat this post negatively

Like I said, it's just my opinion. You're free to do whatever you want

My intentions are to help you
