Sorry to hear you are having bad luck with your Danios. The cause of death is probably due to the water imbalance that hasn't had chance to mature to the right condition ready for your fish. Found an article which might explain in detail.
I will let others offer their inputs too as you don't want to be overwhelmed with information. I am unsure what is the best solution to your current situation but hopefully someone here can! They have helped me a lot.
See home page, and see list of articles for new visitors which might have the answers to your problems. Do not be afraid to ask any other questions!
Re: Fish dying in new tank, recover in bowl water.
the water in 55g gallon is would better for the fish because that tank is 'matured' even though you have add same tap water to both. I wonder if you could extract some the old water from 55g into the smaller tank 10g? Please wait until if other experts agree with this....as I could be wrong.