There is a fish at/in the aquarium.
English bilingual mostly Canadian English
Which is correct? Thanks...

There is a fish in the aquarium.

There is a fish at the aquarium.
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There is a fish in the fish-tank or fish-bowl.

For some reason, 'aquarium' mostly means a public building in which there are many water containers with fish in them, and not the actual fish-tank. This is not the case in my language, where 'aquarium' is the default word for 'fish-tank'. I suppose this is also the case in Turkish?

As for the preposition, most definitely 'in', otherwise the fish tends to die if left in front of the building, i.e. at the aquarium

For some reason, 'aquarium' mostly means a public building

I don't know about "mostly". I've just checked "aquarium" in five different dictionaries, and they all give the tank definition first and the institutional one second, so I am quite happy with the use of "aquarium" in the OP's sentences.

But yes, definitely "in", not "at".

So have I, Ed - the tank definition is first, I know, but I have only ever heard people refer to their fish-tanks as fish-tanks. How should I put it - I am more than happy, and even happier than you, with 'aquarium' meaning 'fish-tank', all the more so that it is the default word in my language. It is just that my experience of its use in English is different for some reason I do not understand...

I guess the fancier the tank or the house is, the more likely you are to call it an "aquarium".

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In Finding Nemo, the Disney animated film, the dentist has an "aquarium" in his home. Well, he would, wouldn't he?

In Finding Nemo, the Disney animated film, the dentist has an "aquarium" in his home. Well, he would, wouldn't he?

Actually, the aquarium is in his office.

Anyone who is remotely into keeping fish as a hobby would use "aquarium." Children who have won a goldfish at a carnival have a fish tank.