A Poorly Fish

It could well be you got a sickly one. Considering how many fish are bred/wild caught in the industry, they just don't have the time to separate off all the slightly below par specimens. They do go through a lot in their journey to your tank, so some are bound to be damaged along the way.

Keep an eye on it for now, and if you ever do decide its time has come, there are two ways to euthanase it.

Put a tub of water in the freezer, and when ice has formed on the surface, break it and put the fish in. Make sure it's dead as danios are temperate water fish and can survive lower temps than tropicals.

Use clove oil (from a pharamcist). Put a bit of tank water in a tub, and place the fish in it. Add 25 drops of clove oil to a small screw topped bottle then add some cold water to the bottle, shake well to emulsify the oil and pour into the tub with the fish. Again make sure it's dead. Because clove oil knocks them out before killing them, leave the fish in there a couple of hours before disposing of the body. It could wake up again if you remove it too soon.
Warning - clove oil stinks. I always use rubber gloves.



PS, good about your ammonia and nitrite.