what is this? HELP

Yes, that is the perfect sign of a fish that is heavily infected with Ich (or also known as Ick). Please read out article on this as it recommends various different treatment methods since some people don’t like using medicine, and others prefer to use heat.

Ich (Ichthyophthirius multifiliis)

Whatever you do, treat the whole tank and don’t remove fish or decorations. You can see it’s already infecting other fish (the one behind the main one that you took a photo of). Give it 2 weeks atleast or treatment and don’t end it early since when it’s at it’s early stages it’s not visible to the naked eye.

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Thank you so much for the replies! So should I go with heat or medicine? And do I need to take anything out of my filter?

You can use heat or the medicine approach, but if you use medicine make sure to take out any carbon in your filter(s) otherwise the carbon will soak it up and it won’t be in the tank for the fish.