Аквариумный термометр показал двадцать пять.

Аквариумный is an adjective meaning for an aquarium / aquarium-related. Russian often uses adjectives this way where English might use a noun as a modifier.

So:

• аквариум = aquarium
• аквариумный термометр = aquarium thermometer

This is the most natural way to say it in Russian.
Термометр аквариума would sound more like the aquarium’s thermometer, which is possible in some contexts but is less like the standard product-type name.

It comes from the noun аквариум (aquarium) plus the adjective-forming suffix -н- / -ный.

So:

• аквариум = aquarium
• аквариумный = aquarium-related, meant for an aquarium

This is a very common Russian pattern:

• школа → школьный = school / school-related
• зима → зимний = winter / wintry
• кухня → кухонный = kitchen / kitchen-related

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is термометр, which is a masculine singular noun. So the past tense must also be masculine singular:

• показал = masculine singular
• показала = feminine singular
• показало = neuter singular
• показали = plural

So:

• термометр показал = the thermometer showed

Yes. Показал is the past tense of the perfective verb показать. Here it means something like:

• showed
• gave a reading of
• registered

In this sentence, it presents the reading as a completed result: the thermometer registered 25.

If you wanted a more ongoing or descriptive sense, Russian might use показывал in some contexts, but in a simple report of a measured result, показал is very natural.

In Russian, after verbs like показать when talking about a thermometer, the word градусов is often omitted if the meaning is obvious.

So:

• Термометр показал двадцать пять. = The thermometer showed twenty-five.
• Термометр показал двадцать пять градусов. = The thermometer showed twenty-five degrees.

Both are natural. The shorter version sounds completely normal when everyone already knows you are talking about temperature.

Because Russian numerals affect the noun that follows them.

With 25, the noun takes the form used after numbers ending in 5–9, 0, or the teens (11–14), which is:

• градусов

Compare:

• 1 градус
• 2, 3, 4 градуса
• 5 градусов
• 21 градус
• 22 градуса
• 25 градусов

So the full phrase is:

• двадцать пять градусов

Russian has no articles. There is no direct equivalent of a or the in normal sentences.

So:

• Аквариумный термометр показал двадцать пять.

can mean:

• The aquarium thermometer showed twenty-five
• An aquarium thermometer showed twenty-five

Which one is intended depends on context.

Yes, it functions as the direct object of показал.

However, for двадцать пять, the nominative and accusative forms are the same in this context, so you do not see any change.

That is why it simply appears as:

• двадцать пять

If a noun is added, you still get:

• показал двадцать пять градусов

because for an inanimate phrase like this, the accusative looks the same as the nominative.

No, Russian word order is fairly flexible, though the given version is very natural and neutral:

• Аквариумный термометр показал двадцать пять.

This is the most straightforward order: subject + verb + object

You might also hear other orders for emphasis, for example:

• Двадцать пять показал аквариумный термометр.
  = It was twenty-five that the aquarium thermometer showed.

But for a textbook-style neutral sentence, the original order is best.

Usually, yes, unless context suggests otherwise.

In everyday Russian, temperature readings are normally understood as degrees Celsius. So if someone says:

• Термометр показал двадцать пять.

people will usually understand 25°C, not Fahrenheit.

The main stresses are:

• аквариу́мный
• термо́метр
• показа́л
• два́дцать пять

A rough pronunciation guide:

• аквариу́мный ≈ ahk-vah-ree-OOM-nyy
• термо́метр ≈ tyr-MOH-myetr
• показа́л ≈ pah-kah-ZAHL
• два́дцать пять ≈ DVAHT-syt' pyat'

The soft sign in пять softens the final consonant, so it is not a hard English t sound.

Yes, absolutely. That would mean:

• The temperature in the aquarium was twenty-five degrees.

It is a different way of expressing the same basic idea.

Compare:

• Аквариумный термометр показал двадцать пять.
  Focus: the instrument’s reading

• Температура в аквариуме была двадцать пять градусов.
  Focus: the temperature itself

Both are correct, but the original sentence specifically talks about what the thermometer showed.