A aquariofilia precisa de um aquário para peixes ornamentais.

Because aquariofilia is a feminine singular noun in Portuguese, so it takes the feminine singular definite article a.

• a aquariofilia = the aquarium hobby / fishkeeping
• Compare:
  • o aquário = the aquarium
  • a aquariofilia = the practice or hobby of keeping aquariums/fish

So a here means the, not a/an.

Aquariofilia refers to the hobby or practice of keeping aquariums, especially with ornamental fish. In English, possible translations include:

• fishkeeping
• aquarium keeping
• the aquarium hobby

It is built from:

• aquário = aquarium
• -filia = fondness/love for something

So the word literally suggests a fondness for aquariums.

Because when precisar means to need, standard Portuguese normally uses de before a noun:

• precisar de algo = to need something

So:

• A aquariofilia precisa de um aquário = Fishkeeping needs an aquarium

A very useful contrast is:

• precisar de + noun
  • Preciso de água. = I need water.
• precisar + infinitive
  • Preciso estudar. = I need to study.

In everyday Brazilian Portuguese, some speakers may omit de in casual speech, but precisar de is the standard form when followed by a noun.

Um is the indefinite article (a/an), while o is the definite article (the).

So:

• um aquário = an aquarium
• o aquário = the aquarium

In this sentence, um aquário means any aquarium of that type, not one specific previously mentioned aquarium.

It means for ornamental fish.

• para = for
• peixes = fish
• ornamentais = ornamental

So um aquário para peixes ornamentais is an aquarium for ornamental fish.

Here, para shows purpose/use: the aquarium is intended for ornamental fish.

Because Portuguese often leaves out the article when speaking generically.

So:

• para peixes ornamentais = for ornamental fish in general

If you said:

• para os peixes ornamentais

that would sound more like for the ornamental fish, meaning specific fish already known in the conversation.

Because in Portuguese, adjectives often come after the noun.

So:

• peixes ornamentais = ornamental fish

This is the normal order in Portuguese. English usually puts the adjective first, but Portuguese often puts it after:

• carro vermelho = red car
• plantas aquáticas = aquatic plants
• peixes ornamentais = ornamental fish

Because the adjective must agree with the noun in number.

• peixe ornamental = ornamental fish (singular)
• peixes ornamentais = ornamental fish (plural)

Here peixes is plural, so the adjective also has to be plural: ornamentais.

The accent in aquário shows the stressed syllable and helps indicate pronunciation.

• aquário is pronounced with stress on á: a-QUÁ-rio

The written accent tells you that the stress is not on the final syllable. This is important in Portuguese because stress placement is not always predictable without accent marks.

Peixes is clearly plural in Portuguese.

• peixe = fish (singular)
• peixes = fish/fishes (plural)

Portuguese marks the plural more explicitly than English does here.

It is understandable, but it sounds a bit formal or dictionary-like. It feels more like a general statement about the hobby than everyday conversation.

More natural possibilities might be:

• Para praticar aquariofilia, é preciso ter um aquário para peixes ornamentais.

  • To practice fishkeeping, you need an aquarium for ornamental fish.

• A aquariofilia envolve manter peixes ornamentais em aquários.

  • Fishkeeping involves keeping ornamental fish in aquariums.

So the original sentence is grammatically fine, but not especially conversational.

In this sentence, precisa means needs because the subject is a aquariofilia, which is third person singular.

Conjugation of precisar in the present tense:

• eu preciso = I need
• você/ele/ela precisa = you/he/she needs
• nós precisamos = we need
• vocês/eles/elas precisam = you/they need

So:

• A aquariofilia precisa... = Fishkeeping needs...

A rough pronunciation guide for a native English speaker is:

a a-kwa-ree-oh-fee-LEE-ah preh-SEE-zah jeh oong a-KWA-ree-oh PAH-rah PAY-sheez or-nah-men-TAH-ees

A few Brazilian Portuguese notes:

• de um often sounds like djum or jũ in connected speech
• ti and di can sound softer in many Brazilian accents, but that does not affect this sentence much
• peixes in Brazil is often pronounced roughly like PAY-shees

This is only an approximation, but it can help you get started.