Flared gills and open mouth indicate something poisoned the fish. This is usually poor water quality or chemicals.
What is the ammonia, nitrite, nitrate and pH of the tank water?
How long has the tank been set up for?
How long have you had the fish for?
What other fish are in the tank?
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Do a 75% water change and gravel clean the substrate every day for a week or until we work out what is going on.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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The following link has information about what to do if your fish get sick. It's long and boring but worth a read when you have some spare time. I recommend printing it out and reading it in bed to help fall asleep
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If your fish ever looks sick or unwell, then the following steps might help. Test the Water and Clean the Tank. Test the water quality for ammonia, nitrite, nitrate and pH and write the results down in numbers. Check it for general hardness (GH) and carbonate hardness (KH) too if you can, but...
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