Some believe this koi carp is Pennywise disguised as a fish to innocently and curiously approach Will, which could be true because when the fish swims down and "disappears," Will's "dad" appears to torment him, hahaha. And well, this coloration in koi is sometimes called "clown," which isn't entirely incorrect, but the most accurate term would be "kohaku," which in the context of koi means "red and white" in Japanese.
And I have no idea who made that Pennywise image, but I'm so grateful they did because I love that "clownfish" joke, so ridiculously, hahaha.