Азотный цикл остановился вплоть до того, что воду меняли.

Азотный is an adjective meaning nitrogen-related or nitrogen in the adjectival sense. It comes from азот = nitrogen.

So:

• азот = nitrogen
• цикл = cycle
• азотный цикл = nitrogen cycle

Russian often uses an adjective where English might use a noun-noun combination. English says nitrogen cycle, but Russian prefers азотный цикл.

Because остановился is the intransitive/reflexive form and means stopped or came to a stop.

Compare:

• остановить что-то = to stop something
• остановиться = to stop oneself / to stop

In this sentence, the subject is азотный цикл, so the idea is that the nitrogen cycle stopped. No direct object is being stopped by someone else, so остановился is the correct form.

The ending -ся marks a reflexive verb. In many cases, it does not mean a literally reflexive action like wash oneself; it often changes the verb into an intransitive one.

Here:

• остановить = to stop something
• остановиться = to stop

So -ся is part of the normal dictionary form of the verb needed here.

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

The subject is цикл, which is:

• singular
• masculine

So the verb must be masculine singular in the past tense:

• цикл остановился

If the subject were feminine, you would get остановилась.
If plural, остановились.

This expression means something like:

• to the point that
• to such an extent that
• so much so that

It introduces an extreme result or consequence.

So the sentence is saying that the nitrogen cycle stopped so severely that they were changing the water.

A rough structure is:

• вплоть до того, что ... = to the point that ...

This is a set phrase, so it is best learned as a chunk.

By itself, вплоть often means something like closely, right up to, or as far as depending on context. In the fixed expression вплоть до, it means up to the point of or all the way to.

Examples:

• вплоть до утра = until morning / right up to morning
• вплоть до мелочей = down to the smallest details

In your sentence, вплоть до того, что is an idiomatic extension of that idea: all the way to the point that...

Because что introduces a subordinate clause here.

Main part:

• Азотный цикл остановился вплоть до того

Subordinate clause:

• что воду меняли

Russian normally uses a comma before что when it introduces a clause like this.

Воду is the accusative singular form of вода.

The verb менять usually takes a direct object in the accusative:

• менять воду = to change the water

So:

• nominative: вода
• accusative: воду

That is why the sentence says воду меняли.

In воду меняли, the subject is left unstated. This is very common in Russian.

The verb form меняли is:

• past tense
• plural

When Russian uses a plural past-tense verb with no subject, it often means:

• they changed the water
• people changed the water
• the water was being changed (in a looser English rendering)

It is an indefinite-personal construction. The speaker either does not know, does not care, or does not need to specify who did it.

Меняли is imperfective past tense. That often suggests:

• repeated action
• ongoing action
• a process
• background information

So воду меняли can suggest they were changing the water or they had to keep changing the water, depending on context.

If you said поменяли воду, that would sound more like a single completed action: they changed the water.

In this sentence, the imperfective form fits well because it presents the water-changing as a consequence or practical measure, not as one sharply completed event.

Not necessarily in a fully literal mechanical sense. In Russian, остановиться can be used quite naturally for processes that stop functioning, stall, or cease to operate normally.

So азотный цикл остановился means the nitrogen cycle stopped working or broke down.

English might sometimes express this more naturally as:

• the nitrogen cycle crashed
• the nitrogen cycle stopped functioning
• the nitrogen cycle broke down

But in Russian, остановился is a normal way to express that kind of stoppage.

A useful breakdown is:

• Азотный цикл остановился = The nitrogen cycle stopped
• вплоть до того, что = to the point that / so much so that
• воду меняли = they were changing the water / the water had to be changed

So the overall pattern is:

[something happened to such a degree] + вплоть до того, что + [result/consequence]

That is the main grammar pattern to notice here.