Hi Anthea, I don't believe you are doing anything 'wrong', it could be a mix of factors.
The first rule of fishkeeping is 'if you don't know why, do a water change' so doing a good one as Ron advised is a good idea.
It is possible that the danios have brought some sort of disease in to the tank with them, it does happen but not always.
Ira may be right about shock causing it but I am wondering if it is the pH which is pretty high for the fish you have. Most shop tanks and established tanks have a lower pH than that. If the fish have come from tanks with lower pH then they will take a lot longer than twenty minutes to acclimatise to the new pH levels. Is 7.5 what the water in the taps is or do you have something in the tank keeping the pH high like a piece of coral rock, shells, or it could be rocks or substrate.
Please do check your ammonia and nitrite tonight - sometimes a newish tank will 'spike' during the night meaning the fish die but during the day the tests are normal.