A Girraffe Hap? I assume the fish is a Nimbochromis type from Lake Malawi?

Anyway, in any Malawi setup, even though the fish may 'grow' together that doesn't mean they will not be territorial and attack each other when they grow up. That's probably why your Girraffe Hap died.

Also, some fish are known to be easily spooked and could just as easily hit himself/herself on the glass/rock/sand/etc and do fatal damage.

Of course, it is even possible to be a disease of some sort though highly unlikely.

Anyway, keep watch on your other fishes, good luck & happy fishkeeping!

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~ John ~
I am a Malawi Fan!