Manta is too big to be seen in lagoon area! I encountered a couple of eagle rays around 200s beach, very lucky but just for one time. And from the deck of our villa in 700s section, we occasionally saw stingrays.
As far as I know, lionfish is a nocturnal animal so I have never seen it even at Conrad. But I saw lots of them when I did a night snorkeling (paid activity) around 700s facing south- highly recommended if you haven't experienced.