How old are the black ocellaris? When I first got mine they wouldnt go into anything, not even a ritteri (or mag if you want to be more correct). I no longer have any nems, but now that the ocellaris are more mature, they have started hosting in a st. thomas mushroom. Its never a sure thing as to what a clown will decide to like, but the most common wild hosts are gigs and mags, but with captive clowns you still never know. If you are buying a nem, I would go with a gig or a mag (if you are confident that your tank will support them.) If you already have I would stick with what you have, and either way I would give the clowns a few months to mature and adapt to their prospective host.