Aunque el acuario ocupa bastante espacio, mi madre dice que el pez la relaja.

Aunque means although / even though. It introduces a contrast:

• Aunque el acuario ocupa bastante espacio = Although the aquarium takes up quite a lot of space
• mi madre dice que el pez la relaja = my mother says the fish relaxes her

Putting aunque at the beginning is very natural in Spanish when you want to present the objection or downside first.

Spanish uses the verb ocupar very often to mean to take up space, time, room, etc.

So:

• ocupar espacio = to take up space
• ocupar mucho tiempo = to take a lot of time

This is a very common and natural Spanish structure.

Bastante here means quite a lot of / a fair amount of.

So bastante espacio means:

• quite a lot of space
• a fair amount of space

It does not have to mean an extreme amount. It often suggests more than is ideal, but not necessarily huge.

Because espacio is being used in a general, uncountable sense here.

Spanish often says:

• ocupa espacio = it takes up space
• ocupa mucho espacio = it takes up a lot of space
• ocupa bastante espacio = it takes up quite a lot of space

You would not normally say ocupa un espacio in this context, because that would sound more like it occupies a particular space/spot rather than it takes up room.

Here que means that and introduces the clause after dice:

• Mi madre dice que... = My mother says that...

In English, that is often omitted:

• My mother says the fish relaxes her

In Spanish, que is normally required after verbs like decir, pensar, creer, etc.

Because pez means fish when it is alive, especially as an animal.

• pez = fish as an animal
• pescado = fish as food

So in an aquarium, you have un pez, not un pescado.

In this sentence, el pez most naturally refers to the fish in the aquarium, meaning a specific fish.

So:

• el pez la relaja = the fish relaxes her

It is not really talking about fish in general as a species. It is talking about that particular pet fish.

La means her.

It is a direct object pronoun, and it refers back to mi madre.

So:

• mi madre dice que el pez la relaja
• literally: my mother says that the fish relaxes her

Spanish very often replaces a repeated noun with an object pronoun like lo, la, los, las.

In Spanish, object pronouns usually come before a conjugated verb.

So:

• el pez la relaja
• not el pez relaja la

This is normal Spanish word order with pronouns.

Compare:

• La veo = I see her
• Lo entiendo = I understand it / him
• El pez la relaja = The fish relaxes her

Because relajar takes a direct object, and the standard direct object pronoun for her is la.

So:

• la relaja = it relaxes her

Not:

• le relaja

In some parts of Spain, you may hear le used where standard grammar expects lo, especially with masculine people, but with a feminine direct object like mi madre, standard Spanish is clearly la.

The subject of relaja is el pez, so the verb must be third person singular:

• el pez relaja = the fish relaxes

Mi madre is not the subject of relaja. She is the object, represented by la.

So the structure is:

• subject: el pez
• object: la = her
• verb: relaja

Yes. Aunque can be followed by either the indicative or the subjunctive, depending on meaning.

Here we have:

• Aunque el acuario ocupa bastante espacio...

The indicative (ocupa) is used because the speaker presents this as a real, known fact: the aquarium really does take up a lot of space.

If the idea were uncertain, hypothetical, or not being presented as an established fact, Spanish could use the subjunctive.

So this sentence uses the indicative because the situation is being stated as true.

Yes. That would also be correct.

• mucho espacio = a lot of space
• bastante espacio = quite a lot of space / a fair amount of space

The difference is mainly nuance:

• mucho sounds more directly like a lot
• bastante can sound slightly softer or more moderate

Both are natural here.

Yes, it sounds natural in Spain Spanish, and also in general standard Spanish.

A few features that are very normal and natural:

• aunque for although
• ocupar espacio for take up space
• decir que for say that
• la relaja for relaxes her

Nothing in the sentence sounds strange or overly formal. It is a normal everyday sentence.