That's a Myco-bacteria common to Rainbows. Especially asian tank raised. In good condition they kick it very quickly. Treat in a quarantine tank with Fungus Cure and anti bacterial feed for 2 weeks . What type rainbows ? HTH Prof M

Edit: Regarding Rainbows in Low PH Mine are currently at 6.3 and breeding like rabbits, So I'm pretty sure they are relatively unconcerned.