Mosquito fish turning black

[Moonweaver]

I have a ton of baby mosquito fish and one of the larger ones are turning black. It was an overnight change and I'm not sure if he's sick or anything.

The last picture is a picture of the rest of them.

 

[fish48]

I don't think it’s turning black, sometimes fish can temporary go darker in colour.
there can be a number of reasons it can be caused by stress, water conditions, or if a fish bumps itself it can also cause it to change colour

 

[Colin_T]

The fish is sick. It could be poor water quality or a muscle wasting disease (Spironucleus).

Check the water quality and do big daily water changes and gravel cleans for a couple of weeks.

If the water is good, then add salt to treat the Spironucleus. Add 4 heaped tablespoons per 20 litres of water. Keep the salt in the water for at least 2 weeks, preferably 4 weeks.

If you have soft water fishes in the tank, move them out so the salt doesn't affect them.