Нитрит вырос вплоть до того, что аквариумист проверил нитрат.

It means to the point that, so much so that, or to such an extent that.

So:

Нитрит вырос вплоть до того, что аквариумист проверил нитрат.
= The nitrite rose to the point that the aquarist checked the nitrate.

This expression often introduces a consequence that feels noteworthy, surprising, or a little extreme.

No. In this sentence, до того is not temporal.

By itself, до того can sometimes mean before that, depending on context. But here it is part of the fixed expression:

вплоть до того, что

In this expression, it means something like as far as the point where... / to the extent that...

So you should understand the whole chunk together, not word by word.

Because the subject is нитрит, which is:

• masculine
• singular

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

• masculine singular: вырос
• feminine singular: выросла
• neuter singular: выросло
• plural: выросли

So:

• Нитрит вырос = Nitrite rose
• Температура выросла = The temperature rose

It is in the accusative. The reason it looks unchanged is that for inanimate masculine singular nouns, the accusative usually has the same form as the nominative.

So:

• nominative: нитрат
• accusative: нитрат

That is why:

• аквариумист проверил нитрат

looks the same as the dictionary form.

Compare with a feminine noun:

• аквариумист проверил воду
  Here the accusative is visibly different from вода.

Because что introduces a subordinate clause.

The main part is:

• Нитрит вырос вплоть до того

and the subordinate clause is:

• что аквариумист проверил нитрат

Russian normally separates this kind of clause with a comma, just as English often separates similar structures in writing less visibly with wording like so much so that...

Both are possible, but they are slightly different in style.

• Нитрит вырос is shorter and can sound natural in technical or hobby contexts, where people understand you mean the nitrite level/concentration.
• Уровень нитрита вырос is more explicit and often clearer, especially for learners.

So a more fully spelled-out version would be:

• Уровень нитрита вырос вплоть до того, что аквариумист проверил нитрат.

That may sound a bit clearer than using the chemical name by itself as the thing that rose.

It is understandable, and in context many speakers will take it to mean checked the nitrate level.

But more explicit or more natural versions in aquarium talk might be:

• проверил уровень нитрата = checked the nitrate level
• сделал тест на нитрат = did a nitrate test
• проверил нитраты = checked nitrates

Which version sounds best depends on the speaker and the exact context. Your sentence is grammatical, but in real-life aquarium discussions people often use more specific wording.

Yes. A common simpler alternative is:

• настолько, что = so much that / to such an extent that

For example:

• Нитрит вырос настолько, что аквариумист проверил нитрат.

This is very close in meaning.

The difference is that вплоть до того, что can sound a bit more emphatic, like it got as far as the point where...

Yes, they could be plural in some contexts, but the singular is possible when talking about the substance or the measured parameter in a general way.

In scientific or hobby language, Russian may use:

• нитрит = nitrite
• нитрат = nitrate

to refer to the substance/reading in general.

But aquarium hobbyists also often say:

• нитриты
• нитраты

especially when thinking of measured compounds or test readings.

So singular here is not wrong; it is just one way of expressing it.

The structure is:

[Subject] + [verb] + [degree/result expression] + [subordinate clause]

So here:

• Нитрит = subject
• вырос = verb
• вплоть до того = degree/result expression
• что аквариумист проверил нитрат = subordinate clause showing the result

A very literal breakdown would be:

• Nitrite rose, up to the point that the aquarist checked nitrate.

That is why the sentence feels like X became so high that Y happened.