Thanks everyone for the replies, much appreciated! So the reason I was asking the question is this. Maybe it's kind of corny, but I wanted to try having 3 pearlscale type butterflies in the 240 - one with red (paucifasciatus), one with orange (xanthurus), and one with yellow (mertensii).
Now, the mertensii that I have seen in person, at least in the last few years, have always had very yellow posteriors, regardless of size, like this:
What I don't like about this fish is the smudge instead of a defined crown (as tcmfish pointed out), and the fact that more often than not the chevrons are distrupted and non-uniform looking, like this (often worse).
Also, the fish just seems more elongated, with a more severe sloping forehead. Again, not so attractive:
Then I saw that LADD has had C. madagaskariensis a few times. As Kevin pointed out, many consider this a synonym of mertensii. Because of this, I was maybe hoping that it would also have the yellow posterior that I have seen in mertensii. But it sounds like the really yellow ones may be regional, expecially if large.