Every so often I receive a comment like this one:
Why do you and your countrymen insist on changing the English language? The plural of aquarium is aquaria, stadium is stadia, etc. etc.
I like to believe that such comments are meant playfully, to get a rise out of the American.
For one thing, the commenter must know that the English language was changing long before there were Americans to speak it.
King James I, for whom the first permanent settlement in Virginia was named, would not have understood the English spoken by his predecessor King Alfred. And Queen Elizabeth II does not speak or write the same English that was spoken by King James I.
Today’s standard British English and standard American English are different dialects. Considering that both diverged from a form of English spoken 400 years ago in England, they remain remarkably similar.
As for the plural of nouns in -um, stadiums and aquariums have become the normal plurals in British periodicals as well as in American. Both dialects do retain the plural stadia in the context of Roman history.
The plural aquaria may still be common among British speakers, but in the US, the Latinate plural is more likely to be used by scientists and serious aquarists than by non-specialists.
The style guide for The Guardian and The Observer states this policy:
Latinate -um neuter endings that are a part of the language (eg stadium) take an -s plural.
Note: This style guide does not use periods with e.g., a fact that seems odd to an American: eg no full points.
The Guardian/Observer guide has separate entries for the following -um nouns:
addendum, plural addendums
aquarium, plural aquariums
memorandum, plural memorandums, not memoranda
referendum, plural referendums, not referenda
It also addresses the datum/data debate:
data takes a singular verb (like agenda), though strictly a plural; no one ever uses “agendum” or “datum.”
The Associated Press Stylebook specifies stadiums as the plural of stadium.
The Chicago Manual of Style advises writers to consult a dictionary for “certain words of Latin or Greek origin such as crocus, datum, or alumna.”
Here are some of the plural choices given in Merriam-Webster Unabridged:
aquarium plural aquariums or aquaria
compendium plural compendiums or compendia
crematorium plural crematoria or crematoriums
encomium plural encomiums or encomia
momentum plural momenta or momentums
stadium plural stadia in the context of ancient Rome; otherwise, stadiums
maximum plural maximums or maxima
memorandum plural memorandums or memoranda
When in doubt, look up the plural of Latinate -um words in a dictionary that targets your standard dialect of English. When a choice is offered, consider the context in which the word is to be used.