When did the girls in h2o start calling it the “moon pool”?

That’s how we all know it but when did they give it that name canonically in the show? Also, a bit of a plot hole, the girls call it the moon pool in h2o as just a way to refer to it I guess but in Mako Mermaids that’s how it’s officially known by all born mermaids for centuries. Was it mermaid instinct to name it that? Or just a bit of messy writing.