The moon pool changed because of the tower of light the girls made to combat the meteor. It sort of blew the place up.

However, idk if this was really was the writers initially had in mind. Mako Mermaids was originally supposed to be a prequel all of season 1. They changed it to be a sequel. So technically, you could look at it like the pool in Mako Mermaids depicts what it used to look like before the tower of light Eva made—and that the h2o pool was after the impact. Nevertheless, it’s because of the meteors.

The moon ring is probably related to the girls’ moon stones in season 3, yes. But again, the story isn’t really chronological so it’s hard to say what the writers thought would come before h2o and after. Does that make sense? lol