It depends on the danios, as ones such as zebra danios, while somewhat small, are extremely active so are recommended to be kept in a 3-4ft tank. They're also subtropical.
If you mean something like Celestial Pearl Danios, that may work, though I'm unsure how they are in hard water. I have hard water and I have neons, but not sure exactly how hard our water is
For black sand you can get a few different kinds at most pet stores. If you are set on getting really cheap kind, tractor supply co. has some black diamond blasting sand.
You'll want to google about it and see if you feel you want to use it. I have a friend who uses it with his bottom dwellers, no problems. But some things I've read..we decided we'd rather just pay a little more.
To make the switch over, you'll want to put all your plants/decorations in a bucket, and your fish in another.
Save a bit of the water to lessen the shock as when switching substrates it's sort of like doing a huge water change.
Take the gravel out, rinse your sand and put it in, give a littel bit to settle.
Refill with some of the water you saved and new dechlorinated tap water. Add decorations and plants, and add fish. Leave lights off to lessen stress.

I think that's about it.