[ThRoewer]

Radianthus ritteri was the "old" incorrect scientific name it was (and still is) traded under.

The "ritteri" species name goes back to a description by Kwietniewski in 1897 as Antheopsis ritteri.

The name it was first described under was: Actinia magnifica by Quoy & Gaimard in 1833. Since this predates all other descriptions the valid species name is and always will be "magnifica"

Fautin then placed it into the genus Heteractis.
So the currently correct scientific name is: Heteractis magnifica (Quoy & Gaimard, 1833).

While the genus may at some point change again, the correct species will always be "magnifica".

So calling it magnifica is correct while ritteri is an incorrect synonym.