Hi, and welcome to Tropical Fish Keeping forum.
I don't know where you are, but if you are using tap water there will probably be chlorine in it, possibly chloramine, and perhaps other "stuff" not good for fish. Chlorine and chloramine will kill fish, and from your posts I suspect this is the cause behind the deaths. A good water conditioner is essential when adding tap water to a fish tank, both at the first and with each regular water change. Not all water conditioners are the same; some only handle chlorine, some chlorine and chloramine, some also detoxify heavy metals, and still others will detoxify ammonia in the tap water. Prime also detoxifies nitrite and nitrate should these be in your tap water. Before you add any other fish please buy a bottle of water conditioner; your local fish store should be able to tell you what is in your local water and thus which conditioner is best.
Danios are hardy, but no fish is that hardy. In addition to the water, without cycling there will be ammonia and then nitrite, both are toxic to fish. Have a read of the info on cycling in the sticky at the head of the Freshwater Aquarium section, here's a link: