Methylene Blue is used to kill bacteria and fungus on fish. It kills filter bacteria too, and stains things blue. Methylene Blue is quite safe and is used to treat some diseases in people. Methylene Blue can increase the oxygen level in fish and this can help with ammonia and nitrite poisoning. However, the best treatment for ammonia/ nitrite poisoning is clean water and getting rid of the ammonia/ nitrite, and increasing aeration/ surface turbulence to maximise the oxygen level in the water.
Malachite Green (aka Victoria Green) is used to treat protozoan parasites like white spot, velvet, costia, chilodonella, trichodina. Malachite Green does nothing to bacteria or fungus, and is carcinogenic (causes cancer). Malachite Green does nothing to increase oxygen levels in fish and actually reduces the oxygen level in water.
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Don't catch fish out and give them salt or chemical baths. Every time you chase and ctch the fish, you damage the fish and allow more areas for harmful disease organisms to enter the body.
If you are using salt or medications, just put the fish in a container and leave it there until treatment is finished after a week or two.
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The fish has fin rot and is sunken in around the head and back.
Fin rot is normally caused by poor water quality that damages the skin and allows harmful bacteria and fungus into the damaged tissue.
The best treatment is clean water (75% daily water changes for a couple of weeks) and salt (1-2 heaped tablespoons of rock salt per 20 litres of water). Keep salt in there for a couple of weeks. If there's no improvement after a few days you might need an anti-bacterial medication (or anti-biotics).
The sunken head is usually lack of food and poor environmental conditions.