Аквариумист проверил воду и аквариумный нагреватель.

Because воду is a feminine noun in the accusative singular, while нагреватель is a masculine inanimate noun, and its accusative singular looks the same as the nominative.

• вода → nominative
• воду → accusative

The verb проверил takes a direct object, so both nouns are in the accusative:

• проверил воду
• проверил нагреватель

But Russian treats masculine inanimate nouns differently from feminine nouns:

• feminine often changes in the accusative: вода → воду
• masculine inanimate often does not: нагреватель → нагреватель

So the second noun is still accusative, even though it looks unchanged.

Because this noun phrase is also a direct object, so it is in the accusative singular.

Since нагреватель is masculine and inanimate, its accusative singular is the same as its nominative singular:

• nominative: аквариумный нагреватель
• accusative: аквариумный нагреватель

The adjective аквариумный agrees with нагреватель in gender, number, and case, so it also stays in the same-looking form here.

If the noun were animate, you would often see a different accusative form. For example:

• Я вижу нового студента.

Here the masculine animate accusative matches the genitive.

Проверил is the perfective past form, so it presents the action as completed: the aquarist checked the water and the heater.

Russian often uses:

• perfective for a completed single action: проверил
• imperfective for process, repetition, or background: проверял

Compare:

• Аквариумист проверил воду. = He checked it, and the checking was completed.
• Аквариумист проверял воду. = He was checking the water / used to check the water / checked it in a process-oriented sense.

In a simple statement about one finished action, проверил is the natural choice.

In the past tense, Russian verbs are formed from the past stem plus endings that show gender and number.

Here:

• проверил = masculine singular past
• проверила = feminine singular past
• проверило = neuter singular past
• проверили = plural

So проверил tells you the subject is masculine singular, which matches аквариумист.

Russian has no articles. It does not use words equivalent to a/an and the in the way English does.

So:

• Аквариумист can mean an aquarist or the aquarist
• воду can mean water / the water
• аквариумный нагреватель can mean an aquarium heater / the aquarium heater

Context tells you which meaning is intended.

If Russian wants to add emphasis similar to English definiteness or indefiniteness, it usually does so through word order, context, intonation, or extra words, not articles.

Аквариумист means an aquarist, someone who keeps or works with aquariums and fish.

It is a noun ending in -ист, a common ending for people associated with an activity, field, or interest.

Examples of similar patterns:

• пианист = pianist
• специалист = specialist
• турист = tourist

Grammatically, аквариумист is a masculine noun. A specifically feminine form can be аквариумистка, though the masculine form may sometimes be used generically depending on context.

Russian often uses adjectives where English might use a noun modifier or a for + noun phrase.

So:

• аквариумный нагреватель = aquarium heater
• literally something like aquarium-related heater

This is very natural in Russian. Another common equivalent is:

• нагреватель для аквариума = heater for an aquarium

Both can work, but аквариумный нагреватель is compact and idiomatic.

Yes. И means and, and here it joins two direct objects of the same verb:

• воду
• аквариумный нагреватель

So the structure is:

• subject: Аквариумист
• verb: проверил
• object 1: воду
• conjunction: и
• object 2: аквариумный нагреватель

In English, this is exactly the same basic idea: The aquarist checked the water and the aquarium heater.

Yes. Russian word order is more flexible than English word order, because case endings show grammatical relationships.

This sentence is in a neutral, straightforward order:

• Аквариумист проверил воду и аквариумный нагреватель.

But other orders are possible, depending on emphasis:

• Воду и аквариумный нагреватель проверил аквариумист.
• Аквариумный нагреватель и воду проверил аквариумист.

These alternatives may sound more marked or literary, or they may highlight a different part of the sentence. For a basic neutral statement, the original order is best.

A good first clue is the ending. Нагреватель ends in a consonant-like written pattern typical of many masculine nouns.

Also, the adjective agrees with it in masculine form:

• аквариумный = masculine singular
• not аквариумная (feminine)
• not аквариумное (neuter)

So the phrase itself shows you the gender:

• аквариумный нагреватель → masculine singular

The main stresses are:

• аквариумИст
• провЕрил
• водУ
• аквариУмный
• нагревАтель

Stress matters in Russian because it can affect both pronunciation and sometimes the vowel quality.

A rough pronunciation guide:

• аквариумист → ak-va-ree-oo-MEEST
• проверил → pra-VYE-ril
• воду → VO-doo? Actually the stress is on the second syllable: va-DOO
• аквариумный → ak-va-ree-OOM-nyy
• нагреватель → na-gri-VA-tyel

If you are learning the sentence actively, it is worth memorizing the stress together with the word.

Yes. Russian often relies on context. In this sentence, because the subject is an aquarist and the other object is an aquarium heater, воду will naturally be understood as the relevant water in that context, probably the aquarium water.

Russian does not always repeat extra detail if the context already makes it clear. So воду by itself is enough.

If someone wanted to be more explicit, they could say something like:

• воду в аквариуме = the water in the aquarium

But that is not necessary here.