Schimbul de apă schimbă nivelul amoniacului.

They are two different words built from the same root:

• schimbul = the change / the exchange
  
  • a noun
  • -ul is the definite article attached to the end, so schimb → schimbul = the change
• schimbă = changes
  • a verb
  • here it is 3rd person singular, matching a singular subject

So in this sentence:

• Schimbul de apă = the water change
• schimbă = changes

This kind of noun/verb pair is very common in Romanian.

It is a noun phrase and it is the subject of the sentence.

Breakdown:

• schimbul = the change
• de apă = of water / water

Together, schimbul de apă means the water change.

In aquarium or fishkeeping language, this is a normal way to say water change.

Because de apă is the normal Romanian pattern for many type/material/content expressions.

Here, apă stays in its basic form after de:

• schimb de apă = water change
• pahar de apă = glass of water
• sticlă de apă = bottle of water

If you used apei, that would be a genitive form and would suggest a different structure. In this sentence, Romanian prefers schimbul de apă, not schimbul apei.

So:

• de apă = the natural expression here
• apei would not be the normal choice in this phrase

Because amoniacului is in the genitive and is also definite.

Base noun:

• amoniac = ammonia

Definite form:

• amoniacul = the ammonia

Genitive/dative definite form:

• amoniacului = of the ammonia / to the ammonia

So:

• nivelul amoniacului literally means the level of the ammonia

In smoother English, you would usually say the ammonia level, but Romanian often uses this noun + genitive structure.

Because Romanian very often uses the definite article in noun phrases where English may sound less definite.

• nivel = level
• nivelul = the level

So:

• nivelul amoniacului = the ammonia level / the level of the ammonia

This is a very normal Romanian structure. English often uses a more compact noun phrase like ammonia level, but Romanian commonly marks the main noun as definite.

The sentence follows a normal subject–verb–object pattern:

• Subject: Schimbul de apă
• Verb: schimbă
• Object: nivelul amoniacului

So literally:

• The water change → changes → the ammonia level

This is one reason the sentence is relatively easy to parse once you spot that the first schimb- word is a noun and the second is a verb.

Because the subject is singular:

• schimbul = the change (singular)

So the verb must also be singular:

• schimbă = changes

If the subject were plural, the verb would change too. For example:

• Schimburile de apă schimbă... = Water changes change...

The letter ă represents a short, central vowel, similar to the a in English about.

So:

• apă sounds roughly like AH-puh
• schimbă sounds roughly like SKIM-buh

A few pronunciation notes:

• sch in schimb sounds like sk, not like English sh
• i in schimb is a normal Romanian ee-ish sound, but short here
• final ă is not silent

Approximate pronunciations:

• schimbul ≈ SKIM-bool
• schimbă ≈ SKIM-buh
• apă ≈ AH-puh
• amoniacului ≈ a-mo-nee-A-ku-luy

Yes, especially in contexts like:

• aquariums
• fishkeeping
• water maintenance systems

It commonly means a water change, meaning replacing some old water with fresh water.

So this is not just a word-for-word combination; it is also a very natural expression in the right context.

Yes, but it is not exactly the same in feel.

• schimbul de apă = a water change as a maintenance action or procedure
• schimbarea apei = the changing/replacing of the water, which feels a bit more like the process itself

In many real situations, both can be understood, but schimbul de apă is especially natural if you mean the routine action known in English as a water change.

Yes. In many contexts, nivelul de amoniac is also very natural and may even sound more idiomatic to some speakers, especially in technical or scientific language.

Compare:

• nivelul amoniacului = the level of the ammonia
• nivelul de amoniac = ammonia level / level of ammonia

Both are understandable and correct in many contexts. The version in your sentence is fully grammatical; it just uses the genitive structure.

Yes, Romanian word order is somewhat flexible, but the given order is the most neutral and straightforward:

• Schimbul de apă schimbă nivelul amoniacului.

That is the standard subject–verb–object order.

Romanian can move parts of a sentence for emphasis, but for a learner, this version is the clearest and safest to use.