Рыбу перекармливали вплоть до того, что малёк стал больным.

Рыбу is the direct object of перекармливали, so it appears in the accusative case.

It is singular grammatically, but Russian often uses a singular noun in a collective sense, especially for animals, fish, or stock being raised. So рыбу here can mean the fish stock or simply the fish as a group, not necessarily one individual fish.

This is a very common Russian pattern: a 3rd person plural verb is used with no explicit subject to mean they, people, or someone in a vague, indefinite way.

So перекармливали here means something like they were overfeeding or, in more natural English, the fish were being overfed.

The imperfective перекармливали presents the action as an ongoing or repeated process. It suggests that the overfeeding was happening over some period of time, and that this process led to the result in the next clause.

If you used perfective перекормили, the focus would be more on a completed act: they overfed the fish. The imperfective fits better when describing a situation that gradually caused a consequence.

The base verb is кормить meaning to feed. With the prefix пере-, it becomes перекармливать / перекормить, which means to feed too much, to overfeed.

So this is not just feeding, but feeding beyond what is healthy or appropriate.

This is a set expression meaning to the point that, so much so that, or even to the extent that.

It introduces an extreme result or consequence. In this sentence, it marks that the overfeeding became so serious that it caused illness.

The preposition до requires the genitive case. The pronoun то therefore changes to того.

So in вплоть до того, что, the form того is there because of case government, not because it has some special separate meaning by itself.

Малёк means fish fry, young fish, or juvenile fish. It is often used as a collective noun, so although it is grammatically singular, it can refer to fry as a group.

That is why the sentence can talk about малёк with a singular verb even if the real-world meaning is plural in English.

Because малёк is a masculine singular noun. Russian verbs in the past tense agree with the subject in gender and number, so the form must be стал.

If the subject were plural, you would get стали. If it were a feminine singular noun, you would get стала.

After стать meaning to become, Russian normally uses the instrumental case for the noun or adjective describing the new state. That is why you get больным.

So the pattern is стать + instrumental: стал больным = became sick

This is a very common structure in Russian.

Yes, малёк заболел would also be possible, but it is not exactly the same in nuance.

Заболел focuses on the moment or event of falling ill. Стал больным emphasizes the resulting state, literally became sick. In this sentence, стал больным works well because it highlights the consequence of the overfeeding.

Russian word order is flexible, and the first position often shows the topic or what the speaker wants to foreground.

Starting with рыбу puts the focus on the fish as the thing affected. It is a natural way to begin the sentence if that is the topic under discussion.

Because the word is pronounced with yo, not plain e. So малёк sounds roughly like ma-LYOK, with the stress on ё.

In many Russian texts, ё is often written as е, so you may sometimes see малек in print, but the correct pronunciation is still with ё.