'dirty' around mouth... could belong to the below scenario:
i once had my lfs guppies in a algae-infested tank (didn't wash the tank for weeks.. that was well before i got serious abt keeping guppy).
i cleaned the tank one day, most of them lost their bright colour and put on a coating of algae on their bodies even their mouths. it's stubbornly stained, i dont think you could remove it in a short period of time. i think can't... never saw that happened
so my suspicion is that your tank is dirty?
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