O termômetro fica acima do aquário.

In Brazilian Portuguese, ficar can mean to be located, to be situated, or to stay somewhere. In this sentence, fica is being used to describe location.

So O termômetro fica acima do aquário means that the thermometer is located above the aquarium.

You could sometimes hear está in similar situations, but fica is very common when talking about where something normally is or where it is positioned.

• fica = is located / sits / stays
• está = is

Here, fica sounds natural because it describes placement.

Fica is the third-person singular present tense form of ficar.

The subject is O termômetro, which is singular, so the verb must also be singular.

Present tense of ficar:

• eu fico = I stay / I am located
• você / ele / ela fica = you / he / she stays, is located
• nós ficamos = we stay
• vocês / eles / elas ficam = you all / they stay

So:

• O termômetro fica... = The thermometer is located...

Acima de means above.

It is a fixed expression:

• acima = above
• de = of / from

So:

• acima do aquário = above the aquarium

A few examples:

• O quadro fica acima da cama. = The picture is above the bed.
• A lâmpada está acima da mesa. = The lamp is above the table.

Because in Portuguese, de + o contracts to do.

So:

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

That means:

• acima de o aquário becomes acima do aquário

This kind of contraction is very common in Portuguese and is usually required.

Examples:

• perto do carro = near the car
• embaixo da mesa = under the table

Portuguese uses definite articles very often, more often than English does.

So even when English might say:

• Thermometer
• Aquarium

Portuguese often says:

• o termômetro
• o aquário

In this sentence:

• O termômetro = the thermometer
• do aquário = of the aquarium / the aquarium

Using the article here sounds normal and natural in Portuguese.

The article tells you: o termômetro.

In Portuguese, nouns have grammatical gender, and termômetro is masculine, so it takes:

• o in the singular
• os in the plural

Examples:

• o termômetro
• os termômetros

Even though many masculine nouns end in -o, not all nouns follow simple ending rules, so the best way is to learn the noun together with its article.

A rough Brazilian Portuguese pronunciation is:

u tehr-MOH-meh-troo FEE-kah ah-SY-mah do ah-kwAH-ree-oo

A few pronunciation notes:

• ô in termômetro is a closed o sound.
• fica has stress on the first syllable: FI-ca.
• acima has stress on ci: a-CI-ma.
• aquário has stress on á: a-QUÁ-rio.

Also:

• Final o in Brazilian Portuguese often sounds like oo.
• The quá in aquário sounds like kwá.

Both can relate to above, but they are not exactly the same.

• acima de = above
• em cima de = on top of / over / directly on

Acima do aquário usually means the thermometer is higher than the aquarium, not necessarily touching it.

Em cima do aquário often suggests it is on top of the aquarium, possibly resting on it.

So if the thermometer is mounted on the wall above the aquarium, acima do aquário is a good choice.

This is the normal Portuguese word order:

subject + verb + location

So:

• O termômetro = subject
• fica = verb
• acima do aquário = location phrase

This is very similar to English:

• The thermometer is above the aquarium.

Portuguese is often flexible, but this order is the most basic and natural here.

Yes. Ficar is a very common verb with several meanings depending on context.

It can mean:

• to stay
• to remain
• to become
• to be located
• to end up

Examples:

• Eu vou ficar em casa. = I’m going to stay home.
• Ela ficou feliz. = She became happy.
• O banco fica na esquina. = The bank is located on the corner.

So in your sentence, the meaning is specifically to be located.

Usually, no—not in normal standard usage for this kind of sentence.

Portuguese normally wants the article here:

• O termômetro fica acima do aquário.

Leaving out o would sound unnatural in most ordinary contexts unless you were speaking in a very special style, like labels, instructions, or headlines.

So for everyday Portuguese, keep the article:

• O termômetro
• not usually just Termômetro