Unfortunately, the problem goes far beyond this particular noun to pretty much all nouns of direct Latin descent. As people study Latin less and less, the use of Latin plural forms is slowly eroding. In general, the more a word has found its way into common usage, the more likely that the English plural has replaced the Latin. For example, "status" is both the singular and plural form in Latin, but "statuses" is common even though it sounds awkward. We all speak of "an agenda", when actually "some agenda" is correct in the Latin sense; one of a group of agenda is an "agendum". Similarly, one item of data is a datum, but we rarely say "the data are on the way." I don't see "fora" in place of "forums" except in forums that I would never join. :)
On the other hand, we generally say "appendices", I suspect primarily because "appendixes" sounds funny. I see both "curricula" and "curriculums". Further, though, I never see "genuses", always "genera", and likewise for "quantums" vs. "quanta".