Fish upside down and spasming

one of my 5 zebra danios was floating around upside down and going mad spasming every so often this evening. I caught him in the net for a while as I though I may have stunned him while moving a plant. but he didn't recover. I took him out and put him out of his misery just to be sure it wasn't something the other fish might catch or anything.

The ammonia levels in the tank have just dropped back to zero, nitrite and nitrate are a bit higher than last time I test but not much at all. I have been changing the water, 4L every second day as though its not fully cycled I didn't want the level to build up too much with the fish in the tank.

The other fish look ok, react if I go near them and all group together, which this one did not. I had thought from the start he might be a bit sick and never noticed all 5 feeding at once, I assumed it was this guy usually missing.

Any guesses, might he have been sick when I got him (Sun 19th) or should I be worried about the other fish too?