Is this the Coral Manta?

https://i.imgur.com/ov2wIF7.png
Check out the Patch 6.3's patch note! I am almost certain that this is the Coral Manta from Ocean Fishing!!!

https://i.imgur.com/AvUuzUW.png
So finally, apart from the Stonescale (which was the only 5-star fish that can be placed in the tank, and many people considered it ugly), they're adding another legendary 5-star Ocean Fishing fish to the fish tank, right? I'm happy! I certainly hope they can add the Sothis in the future!

Btw, I'm really glad I kept all those 5-star fish in my retainer! Now I can go back there and put my manta in the tank! :)

It was, yes.