Not sure what to tell you, but is sounds like mr chromis is doing very poorly, i would remove him.
As for what to do w him, thats up to you. you can choose to let him be or you can assist him.
however its your choice
--Renee provided you similar choices as well.
Euthanasia in Fish
Euthanasia is the event of a gentle painless death. In mammals, euthanasia is usually accomplished by an overdose of an anesthetic agent. The American Veterinary Medical Association (AVMA) Panel on Euthanasia lists acceptable methods of euthanasia for fish. The method accepted by fish veterinarians is an overdose of the anesthetic tricaine methanesulfonate (MS-222). Triple2 is available in small hobbyist size aliquots from a few online aquatic vendors. Follow the manufacturers guidelines for either anesthesia or euthanasia doses. A quick point about Triple2 for fish euthanasia, be aware it quickly depresses the pH of your water. To offset this pH drop add sodium bicarbonate to the water to ensure a less stressful passing.
Two other methods have been suggested but are considerably more difficult to perform; stunning (by a blow to the head) and decapitation. Hypothermia (chilling, freezing) is not considered humane when used as the sole method of euthanasia because the animal is not rendered insensible to pain rapidly enough. If you cant get Triple2 or don’t want to decapitate your fish, here are a few alternatives.
Euthanasia of Fish at Home
Clove oil works very well in fish anesthesia, and is available from specialty stores or pharmacies. Clove oil is not water-soluble but can be rendered soluble by first dissolving it in ethanol, then using this stock diluted in cold water (about 1 ml per gallon, or 1/4 teaspoon per gallon). Submerge the fish in a bag of water and add 1 capful every 3-4 minutes. Clove oil works fairly quickly, as you will notice its effects, by the fish slowing its respiration, then loose buoyancy or lay sideways, then becomes motionless.
Next, carbon dioxide gas can be used as euthanasia for fish. To generate carbon dioxide in water, dissolve 2 or more tablets/liter (8 tablets/gallon) of Alka-Seltzer® tablets with the fish submerged in a covered bowl or bag. Try to keep the bubbling away from its gills while its falls asleep.
Be aware that the fish undergoing euthanasia will initially exhibit a bout of hyper-excitement as their nerves are stimulated. Lastly, after you euthanize your fish, place it in the freezer to ensure death.
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