Learn Hardcore Russian: В гостиной стоит аквариум, где плавают рыбки и маленькая черепаха. - In the living room there is an aquarium where fish and a little turtle are swimming.

The basic word is гостиная (living room), feminine, nominative case.
After в meaning “in/at (location)” you normally use the prepositional case:
- гостиная → в гостиной (“in the living room”)

If it were motion into the living room, you would use the accusative:

в гостиную – “(go) into the living room”

So:

гостиная – (subject form) “the living room”
в гостиной – “in the living room”
в гостиную – “into the living room”

Why is it стоит аквариум and not something like есть аквариум or находится аквариум?

Russian often uses verbs like стоять (to stand), лежать (to lie), висеть (to hang) as neutral “there is” verbs for objects, especially furniture and things in a room.

стоит аквариум literally: “an aquarium stands”
In practice: “there is an aquarium (standing there)”

Alternatives:

В гостиной есть аквариум. – “There is an aquarium in the living room.”
- Neutral existence, not emphasizing position.
В гостиной находится аквариум. – “An aquarium is located in the living room.”
- More formal / neutral “is located”.
В гостиной стоит аквариум.
- Sounds very natural for a physical object that literally stands somewhere.

In everyday speech about where things are in a room, стоит is very common and sounds more vivid/natural than есть or находится.

Could we say Аквариум стоит в гостиной instead of В гостиной стоит аквариум? Is there a difference?

Yes, you can say Аквариум стоит в гостиной, and it’s grammatically correct.

В гостиной стоит аквариум.
- Starts with the location. This emphasizes the room and what is in it.
- Roughly: “In the living room there is an aquarium.”
Аквариум стоит в гостиной.
- Starts with аквариум. This emphasizes the aquarium and where it is.
- Roughly: “The aquarium is in the living room.”

Both are natural; the choice is about focus:

Talking about the living room → start with В гостиной…
Talking about the aquarium → start with Аквариум…

What exactly is где doing here? Is it a question word?

In this sentence где is not a question word. It’s functioning like a relative word, introducing a subordinate clause:

…аквариум, где плавают рыбки и маленькая черепаха.
Literally: “…an aquarium where fish and a little turtle are swimming.”

So here:

где ≈ “where (in which)”
It refers back to аквариум.

English would usually say:

“…an aquarium where fish and a small turtle swim” or
“…an aquarium in which fish and a small turtle swim.”

Why is there a comma before где?

The comma marks the beginning of a subordinate clause that describes аквариум:

Main clause: В гостиной стоит аквариум
Subordinate clause: где плавают рыбки и маленькая черепаха

Russian normally separates a relative / subordinate clause from the main clause with a comma, much like English:

“There is an aquarium where fish and a little turtle swim.”

So the comma is required by punctuation rules when you have a clause introduced by где that describes a noun.

Why are рыбки and маленькая черепаха in the nominative case? Shouldn’t they be objects “in the aquarium”?

They are in the nominative case because they are the subjects of the verb плавают (“swim”).

Look at the clause:

где плавают рыбки и маленькая черепаха

Ask: “Who/what is swimming?” → “рыбки и маленькая черепаха” → subjects → nominative.

Their being in the aquarium is only implied by где, which refers to the aquarium. The grammar of the clause itself is just:

“(There) swim fish and a little turtle.” → subjects in nominative + verb.

Why is it рыбки and not just рыбы?

Рыбки is the diminutive form of рыбы:

рыба – fish (singular)
рыбы – fish (plural)
рыбка – little fish / cute fish (singular, diminutive)
рыбки – little fish / cute fish (plural, diminutive)

Using рыбки instead of рыбы often adds a feeling of:

small size,
affection or cuteness,
a more “homey” or “friendly” tone.

So рыбки matches the cozy, domestic context (a home aquarium) better than the neutral рыбы.

Why is it маленькая черепаха and not something like маленькаяя or маленький черепаха?

Маленькая is the correctly declined form of the adjective маленький (“small, little”) for a feminine singular nominative noun.

черепаха – feminine, singular, nominative
Adjective must agree in gender, number, and case:
- Masculine: маленький мальчик
- Feminine: маленькая черепаха
- Neuter: маленькое окно
- Plural: маленькие дети

So:

маленький черепаха – wrong (mismatched gender)
маленькаяя – not a Russian form; only one я is used.
маленькая черепаха – correct agreement (fem. sg. nom. + fem. sg. nom.)

Why is the verb плавают plural?

Because the subject of the clause is plural overall:

Subject: рыбки и маленькая черепаха
- рыбки – plural
- черепаха – singular
- together: compound subject → treated as plural

Russian, like English, uses a plural verb with a compound subject joined by и (“and”):

Рыбки и маленькая черепаха плавают.
- “The fish and the little turtle are swimming.”

What’s the difference between плавать / плавают and плыть / плывут?

Both verbs mean “to swim,” but they differ in aspect/type of movement:

плавать (here: плавают) – multidirectional, repeated, or generalized swimming.
- “to swim (around), to be able to swim, to swim habitually / in various directions”
- Good for describing how fish move around in an aquarium in general.
плыть (3pl: плывут) – unidirectional, one specific movement in one direction.
- “to be (in the act of) swimming in one direction / floating”

In this sentence, we’re describing the general, ongoing movement of the fish and the turtle in the aquarium, so плавают (from плавать) is the natural choice.

Could we say …аквариум, в котором плавают рыбки и маленькая черепаха instead of …аквариум, где плавают…?

Yes, that’s also correct and maybe even a bit more formal/explicit:

…аквариум, где плавают рыбки и маленькая черепаха.
- “…an aquarium where fish and a little turtle swim.”
…аквариум, в котором плавают рыбки и маленькая черепаха.
- “…an aquarium in which fish and a little turtle swim.”

Где is shorter and very common in speech.
В котором emphasizes the preposition в (“in”) and sounds slightly more formal or written, but both are natural.

What are the genders of the nouns in this sentence, and do they affect the forms we see?

Yes, their genders affect adjectives and verb forms. Here are the key nouns:

гостиная – feminine
аквариум – masculine
рыбка (plural рыбки) – feminine
черепаха – feminine

How gender shows up:

В гостиной – the ending -ой is the prepositional feminine singular form of гостиная.
маленькая черепаха – маленькая is the feminine singular nominative form of the adjective to match черепаха.
стоит аквариум – verb стоит is 3rd singular (works for all genders in present tense); gender doesn’t change the form here.
плавают рыбки и маленькая черепаха – плавают is 3rd plural, because the whole subject is plural. Gender doesn’t affect this plural form, only number does.