O nitrito vira nitrato no biofiltro.

Vira is the 3rd person singular of virar.

In this sentence, virar means to become / to turn into:

• O nitrito vira nitrato = Nitrite becomes nitrate / Nitrite turns into nitrate

This is a very common use of virar in Brazilian Portuguese when one thing changes into another.

Examples:

• Água vira gelo. = Water turns into ice.
• A lagarta vira borboleta. = The caterpillar becomes a butterfly.

Because the subject is O nitrito, which is singular.

So the verb must also be singular:

• o nitrito → singular
• vira → singular

Compare:

• O nitrito vira nitrato. = Nitrite becomes nitrate.
• Os compostos viram gás. = The compounds become gas.

O is the masculine singular definite article, meaning the.

Portuguese often uses articles more often than English does, including with scientific or general nouns.

So:

• o nitrito = the nitrite
• o nitrato = the nitrate
• o biofiltro = the biofilter

Even when English might say just nitrite, Portuguese may naturally say o nitrito.

After verbs like virar when they mean become / turn into, Portuguese often uses the new state or role without an article.

So:

• O nitrito vira nitrato.

This is similar to:

• Ele virou médico. = He became a doctor.
• A água virou vapor. = The water became vapor.

You can sometimes find articles in other contexts, but in a straightforward scientific statement like this, nitrato without an article sounds natural.

No is a contraction of:

• em = in
• o = the

So:

• em + o = no

Therefore:

• no biofiltro = in the biofilter

This kind of contraction is extremely common in Portuguese:

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

Examples:

• na água = in the water
• nos tanques = in the tanks

Both can exist, but biofiltro is a standard noun meaning biofilter.

Portuguese often forms words with bio- just like English does:

• biologia
• biodegradável
• biofiltro

So biofiltro is simply the normal compact term here.
Filtro biológico may also be possible in some contexts, but biofiltro is concise and very natural, especially in technical or aquarium-related language.

The present tense is commonly used in Portuguese for general truths, processes, and scientific facts.

So:

• O nitrito vira nitrato no biofiltro.

means this is what generally happens in that process, not necessarily only right now at this exact moment.

English does the same thing:

• Water boils at 100°C.
• Nitrite becomes nitrate in the biofilter.

Yes. In Brazilian Portuguese, virar is very common and natural in everyday speech.

Other ways to say become include:

• tornar-se — more formal
• ficar — often means to get / become for temporary states
• transformar-se em — to transform into

Compare:

• O nitrito vira nitrato. — very natural
• O nitrito se torna nitrato. — more formal/technical
• O nitrito se transforma em nitrato. — emphasizes transformation

In a sentence like this, vira sounds perfectly normal.

Yes, Portuguese word order is somewhat flexible.

The basic order here is:

• O nitrito (subject)
• vira (verb)
• nitrato (complement)
• no biofiltro (location)

You could also say:

• No biofiltro, o nitrito vira nitrato.

This puts more emphasis on where the process happens.

But the original order is the most neutral and straightforward.

A rough pronunciation guide in Brazilian Portuguese is:

oo nee-TREE-too VEE-rah nee-TRAH-too noo byo-FEEL-troo

A few notes:

• nitrito stress: tri → ni-TRI-to
• nitrato stress: tra → ni-TRA-to
• vira sounds like VEE-ra
• biofiltro is roughly byo-FEEL-tro, with stress on fil

The r in vira is usually a light tap in Brazilian Portuguese, somewhat like the Spanish single r.

It sounds technical but natural.

The vocabulary is scientific:

• nitrito
• nitrato
• biofiltro

But the grammar itself is very normal, especially the use of virar.

So this is the kind of sentence you might see in:

• aquarium care
• water treatment
• biology explanations
• educational material

It is not unusually formal, but it does belong to a scientific context.