Commiserations. When I lived in London I never had any problem raising Danio Fry.
A few years ago I moved to Bedfordshire and found they very rarely bred and when they did, all but a couple of the fry soon died.
I eventually discovered that the water was the problem. In London water is very hard but it is temporary hardness and, in a tank with a heater, water gets softer with time.
In Bedfordshire my water is even harder and it is permanent hardness.
Solution was to get a rain butt and fill the tank with rainwater. They now breed most weeks in the community tank.
I have been experimenting with community tank breeding of danios, so I don't know what they are yet. they could be Kerri, Abolineatus, Roseus, Kyathit striped, Kyathit spotted, choprae, hikari blue or yellow, nigrofasciatus or hybrids of these or between these and my single rerio or goldring. Favourite is Kerri Roseus or Abolineatus and hybrids of all three.
Basically I have an external filter with the eheim large inlet and outlet and harvest the eggs and fry from the filter. You don't get as may as when you breed in a breeding tank but I have so many plants I can't catch them and even when I have I find that putting them in a breeding tank causes prolonged sulking.
It's quite difficult to spot the eggs among the muck but very rewarding.
PS the only ones I bred succesfully in the tap water were Kerri. along with a Kerri-Rerio hybrid which looked spectacular!
The fry are very tolerant to water temperature extremes and dirty tanks so my advice is to get a water testing kit and check out the hardness.
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