Why are your fish turning black and dying with their mouths locked open?

Fish turning black and dying with their mouths locked open is often a sign of ammonia poisoning. Ammonia is a toxic chemical that can build up in the water of an aquarium if the water is not properly filtered or changed regularly. When ammonia levels get too high, it can cause fish to become stressed, their gills to turn red or black, and their mouths to lock open. Eventually, the fish will die from suffocation.

Here are some steps you can take to prevent ammonia poisoning in your aquarium:

* Test the water regularly for ammonia levels. Ammonia levels should be kept below 0.25 parts per million (ppm).

* Change the water in the aquarium regularly. The frequency with which you need to change the water will depend on the size of the aquarium, the number of fish in it, and the type of filter you are using. A good rule of thumb is to change 25% of the water every week.

* Use a water filter. A water filter can help to remove ammonia and other toxins from the water.

* Add live plants to the aquarium. Live plants can help to absorb ammonia and other toxins from the water.

If you see any of the signs of ammonia poisoning in your fish, take action immediately to reduce the ammonia levels in the water. If the ammonia levels are too high, you may need to move the fish to a different aquarium or pond.