O aquarista pratica a aquariofilia.

O is the masculine singular definite article, usually translated as the.

In this sentence, o aquarista can mean:

• the aquarist in a specific context, or
• the aquarist in a generic sense, meaning an aquarist / aquarists in general

Portuguese often uses the definite article in places where English might use a/an or no article at all.

Compare:

• O aquarista pratica a aquariofilia. = a general statement about an aquarist / the aquarist
• Um aquarista pratica a aquariofilia. = one aquarist, any aquarist
• Aquarista pratica aquariofilia. = possible in very telegraphic style, but less natural as a normal sentence

Because word endings do not always determine grammatical gender in Portuguese.

Aquarista is one of many nouns ending in -ista, and these are usually the same form for both men and women. The article tells you the gender:

• o aquarista = the male aquarist
• a aquarista = the female aquarist

This is very common:

• o artista / a artista
• o jornalista / a jornalista
• o dentista / a dentista

So the -a ending here does not make the word feminine.

Yes. If you are talking about a woman, you would say:

• A aquarista pratica a aquariofilia.

The noun aquarista stays the same. Only the article changes:

• o for masculine
• a for feminine

If you add adjectives, they usually agree in gender:

• O aquarista dedicado
• A aquarista dedicada

Because pratica is the conjugated form of the verb praticar in the present indicative, third person singular.

The subject is o aquarista, which is singular, so the verb must also be singular:

• eu pratico
• você / ele / ela pratica
• nós praticamos
• eles / elas praticam

So:

• O aquarista pratica... = The aquarist practices...

If you used praticar, that would be the infinitive, like to practice, and it would not work as the main verb in this sentence.

Yes, it is grammatically correct and understandable.

Praticar is often used for:

• sports
• activities
• disciplines
• hobbies

For example:

• praticar natação
• praticar yoga
• praticar um esporte

With aquariofilia, it sounds a bit formal or technical, but it is still fine.

In everyday Brazilian Portuguese, people might also say:

• Ele é aquarista.
• Ele gosta de aquarismo.
• Ele pratica aquarismo.

So the sentence is correct, but it may sound slightly more formal than casual speech.

That a is the feminine singular definite article, because aquariofilia is a feminine noun.

So:

• a aquariofilia = the aquarium hobby / the practice of keeping aquariums

Portuguese often uses articles with abstract nouns, fields, and activities more often than English does.

So this structure is normal:

• pratica a aquariofilia
• estuda a biologia
• aprecia a música clássica

Also, this a is just an article. It is not a contraction here, because there is no preposition involved.

They are related, but they are not the same kind of word.

• aquarista = the person
• aquariofilia = the activity, hobby, or field

So:

• O aquarista = the aquarist
• A aquariofilia = aquarium keeping / the aquarium hobby

A useful way to think about it is:

• aquarista = someone who does it
• aquariofilia = the thing being done

A helpful approximate pronunciation in Brazilian Portuguese is:

• aquarista → a-kwa-RIS-ta
• aquariofilia → a-kwa-rio-fi-LI-a

A few pronunciation notes:

1. qu before a sounds like kw

   • so aqua- sounds like akwa-

2. In normal Brazilian pronunciation, unstressed o often sounds like u

   • so O aquarista may sound closer to u aquarista

3. The stressed syllables are:

   • aquaRISta
   • aquariofiLIa

So the whole sentence may sound roughly like:

u akwa-RIS-ta PRA-ti-ka a akwa-rio-fi-LI-a

It is a correct word, but it can sound a bit specialized or formal.

In Brazil, aquarismo is often more common in everyday use.

So you may also hear:

• O aquarista pratica o aquarismo.

Both are correct, but the nuance is roughly:

• aquariofilia = a more technical or formal term
• aquarismo = often more common in casual usage

If you are learning Portuguese for real-life conversation in Brazil, it is useful to recognize both words.