O aquarista mede a dureza da água.

Portuguese usually sounds more natural with articles in places where English often leaves them out.

• O aquarista = the aquarist
• a dureza = the hardness
• da água = of the water

So Portuguese prefers the fuller structure here.
Without the articles, the sentence would sound unnatural in most normal contexts.

Aquarista means someone who keeps, manages, or is involved with aquariums or fish tanks—an aquarist.

It can refer to:

• a hobbyist
• a specialist
• someone who takes care of aquarium water, fish, plants, etc.

Not necessarily. Aquarista is one of those nouns that often keeps the same form for both men and women.

So you can have:

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

The article changes, but the noun usually stays the same.

Mede is the 3rd person singular present tense of the verb medir (to measure).

So:

• eu meço = I measure
• você/ele/ela mede = you/he/she measures

In this sentence, o aquarista mede means the aquarist measures or the aquarist is measuring, depending on context.

In Portuguese, the verb medir works very much like to measure in English: it takes a direct object.

So:

• medir algo = to measure something

Here, the thing being measured is:

• a dureza da água = the hardness of the water

So the structure is:

• O aquarista = subject
• mede = verb
• a dureza da água = object

Yes. Dureza literally means hardness.

In this sentence, it refers to water hardness, which is a common scientific or aquarium-related term. It does not mean that the water is physically hard like a rock. It refers to the mineral content of the water.

So this is a technical use of dureza.

Because dureza is a feminine noun in Portuguese.

So:

• a dureza = the hardness

You can often recognize this from dictionary entries, but you should not rely only on word endings. In this case, you simply learn it as:

• a dureza

Because da is a contraction of:

• de + a = da

So:

• a dureza da água literally comes from a dureza de a água
• but Portuguese contracts that to a dureza da água

This is very common.

Other similar contractions:

• de + o = do
• de + os = dos
• de + as = das

Because Portuguese expresses this idea as the hardness of the water:

• dureza de algo = hardness of something

So:

• a dureza da água = the hardness of the water

This is similar to English of-phrases.

Yes, and that is completely normal in Portuguese.

You say:

• a água

Even though água begins with a, it is still a feminine noun, so it takes the feminine article a.

This happens with some feminine nouns beginning with a stressed a sound:

• a água
• a alma
• a águia

So although it may look unusual to an English speaker, it is correct.

The accent mark in água shows the stressed syllable:

• Á-gua

The stress falls on the first syllable.
It also helps show the correct pronunciation and distinguishes the written form according to Portuguese spelling rules.

Yes. The basic order is the same:

• O aquarista = subject
• mede = verb
• a dureza da água = object

So it follows a normal subject–verb–object pattern, which makes this sentence fairly straightforward for English speakers.

Yes. The Portuguese simple present can cover both ideas depending on context.

So mede can mean:

• measures (habitually / generally)
• is measuring (in some contexts)

If you want to be very explicit about an action happening right now in Brazilian Portuguese, you often use:

• O aquarista está medindo a dureza da água.

That means The aquarist is measuring the hardness of the water.

Yes. It sounds natural, especially in a scientific, technical, educational, or aquarium-related context.

It is a clear, standard sentence and a good example of:

• article use
• present tense
• a direct object
• de + a = da contraction

So it is a very useful sentence for learners.