O cardume está no aquário com o cascalho.

Because cardume is a collective noun. It means a school/shoal of fish as one group.

So in Portuguese, the subject is grammatically singular:

• O cardume está... = The school of fish is...

That is why the verb is está, not estão.

If you wanted to talk about the individual fish instead, you could say:

• Os peixes estão no aquário... = The fish are in the aquarium...

Portuguese usually uses estar for location:

• O cardume está no aquário. = The school of fish is in the aquarium.

Use ser (é) for identity, definition, or permanent characteristics, not for physical location.

Compare:

• É um cardume. = It is a school of fish.
• Está no aquário. = It is in the aquarium.

So here está is the natural choice because the sentence tells you where the school of fish is.

No is a contraction of:

• em = in/on/at
• o = the

So:

• em + o = no

That means:

• no aquário = in the aquarium

This kind of contraction is very common in Portuguese.

Other examples:

• na = em + a
• nos = em + os
• nas = em + as

In this sentence, aquário is a masculine singular noun, so it takes o:

• o aquário = the aquarium

That is why em + o becomes no.

There is not always a perfect rule for noun gender in Portuguese, so often you simply learn the noun together with its article:

• o aquário
• o cardume
• o cascalho

Learning nouns with their articles is one of the best habits for Portuguese.

In normal Portuguese, O cardume sounds more natural here than just Cardume.

Portuguese uses definite articles more often than English does. With a common noun as the subject, the article is usually expected:

• O cardume está no aquário.

Saying Cardume está no aquário would sound unnatural in most everyday contexts, unless it were some special style such as a headline, label, or very unusual context.

Literally, it means with the gravel.

In most natural readings, it describes the aquarium:

• no aquário com o cascalho = in the aquarium with the gravel

So the idea is that the aquarium has gravel in it.

However, grammatically, there is a little potential ambiguity, because com o cascalho could seem attached to the whole phrase before it. In real life, though, listeners will usually understand that the gravel belongs to the aquarium setting, not that the fish are somehow accompanied by gravel in a special way.

If you want to make it clearer, you could say:

• O cardume está no aquário que tem cascalho.
• O cardume está no aquário com cascalho no fundo.

Because com normally does not contract with the definite article in modern standard Portuguese.

So you say:

• com o cascalho = with the gravel

not a single contracted form like co.

You will see contractions with some prepositions, but not all prepositions behave the same way. For example:

• de + o = do
• em + o = no
• a + o = ao

But:

• com o stays com o

Cascalho means gravel or small stones/pebbles, especially the kind used on the bottom of an aquarium, path, or ground.

In this sentence, it most naturally refers to the gravel at the bottom of the aquarium.

Depending on context, Portuguese can also use related words like:

• pedrinhas = little stones
• brita = crushed stone/gravel, more common in construction contexts

For an aquarium, cascalho is very natural.

Yes. The accents matter for both pronunciation and sometimes meaning.

• está has an accent to mark the stressed final syllable: es-TÁ
• aquário has the stress on quá: a-QUÁ-rio

In the case of está, the accent also helps distinguish it from esta, which can mean this (feminine):

• esta = this
• está = is

So the accent is definitely not optional.

The lh sound in Portuguese is a special consonant, similar to the lli in some pronunciations of million, though not exactly the same in every accent.

So cascalho sounds roughly like:

• cahs-CAH-lyo

Very roughly, not perfectly.

A few other examples with lh:

• filho = son
• mulher does not have lh, so it is different
• trabalho = work

It is a common sound in Portuguese, so it is worth practicing.

Yes, but the meaning changes slightly.

• O cardume está no aquário com o cascalho.
  = The school of fish is in the aquarium with the gravel.

• Os peixes estão no aquário com o cascalho.
  = The fish are in the aquarium with the gravel.

Cardume emphasizes the fish as one group moving together.
Peixes just means fish in general.

Also notice the grammar change:

• O cardume está ... because cardume is singular
• Os peixes estão ... because peixes is plural

The sentence follows a very common Portuguese pattern:

• O cardume = subject
• está = verb
• no aquário com o cascalho = location phrase

So it is basically:

• Subject + verb + place

This is the most neutral and natural order.

Portuguese can change word order for style or emphasis, for example:

• No aquário com o cascalho, está o cardume.

But that sounds more marked or literary. For a learner, the original order is the safest and most natural choice.