From what i can see it looks like hemorrhagic septicemia. Are the fins red streaked as well? if so then it is bacterial. This is caused by stress and poor water quality. Even though the water is now perfect bacteria is ever present in the water and if a fish gets stressed then this lowers the immune system allowing the bacteria to invade and septicemia is where the fish looks bruised from under the skin.
All i can suggest is an anti bacterial remedy to kill any bacteria in the water and salting the water to .2% but it can progress to ulcers and dropsy.
Can you quarantine the fish so you can medicate it? More water changes may help.

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