Аквариумный грунт оказался слишком тёмным.

Because after оказаться in the sense of to turn out to be / to prove to be, Russian normally uses the instrumental case for the thing’s resulting or discovered quality.

So:

• грунт оказался тёмным = the substrate turned out to be dark
• not usually грунт оказался тёмный

This is a very common pattern:

• Он оказался прав. = He turned out to be right.
• Комната оказалась маленькой. = The room turned out to be small.

Here, тёмным is the instrumental singular masculine/neuter form of тёмный.

Оказался is the past tense masculine singular form of оказаться.

In this sentence, it means:

• turned out to be
• proved to be
• was found to be

So the idea is not just the substrate was too dark, but rather it turned out that the substrate was too dark.

That often implies some discovery after seeing or using it.

Because it agrees with the subject, грунт, which is a masculine singular noun.

Past tense verbs in Russian change for gender and number:

• оказался — masculine singular
• оказалась — feminine singular
• оказалось — neuter singular
• оказались — plural

Since грунт is masculine, we get оказался.

Грунт is a broad word. Depending on context, it can mean:

• soil
• ground
• base material
• primer in some technical contexts

In an aquarium context, аквариумный грунт usually means the material at the bottom of the tank, such as:

• gravel
• substrate
• aquarium soil

So in this sentence, substrate is probably the most natural translation.

Аквариумный means aquarium or for an aquarium.

It is an adjective formed from аквариум (aquarium) + the adjective ending -ный.

It agrees with грунт:

• masculine
• singular
• nominative

So:

• аквариумный грунт = aquarium substrate / substrate for an aquarium

This is a normal way Russian forms adjectives from nouns.

Слишком means too in the sense of more than is acceptable / more than desired.

So:

• слишком тёмный = too dark
• очень тёмный = very dark

That is an important difference:

• очень just strengthens the description
• слишком suggests a problem

So the sentence means the substrate was darker than wanted, not simply very dark.

Yes, you can, but the meaning is slightly different.

• Аквариумный грунт был слишком тёмным = The aquarium substrate was too dark
• Аквариумный грунт оказался слишком тёмным = The aquarium substrate turned out to be too dark

The version with оказался adds the idea of discovery or unexpected realization.

So if someone bought it and later saw that it was darker than expected, оказался fits very well.

Yes. Ё is often written as е in normal Russian writing, even though the pronunciation is different.

So:

• тёмный and темный are the same word
• тёмным and темным are the same form

In careful learning materials, dictionaries, and textbooks, ё is often shown. In everyday texts, it is often omitted.

The dictionary form is тёмный.

Here are the relevant forms:

• тёмный — masculine nominative singular
• тёмным — instrumental singular masculine or neuter

Since грунт is masculine singular and the adjective follows оказаться, we get тёмным.

The stresses are:

• аквариу́мный
• грунт
• оказа́лся
• сли́шком
• тёмным

So the full sentence is pronounced:

аквариу́мный грунт оказа́лся сли́шком тёмным

This is useful because Russian stress is unpredictable and can affect pronunciation a lot.

Yes. Russian word order is flexible, though the neutral order here is very natural:

• Аквариумный грунт оказался слишком тёмным.

You could also rearrange it for emphasis, for example:

• Слишком тёмным оказался аквариумный грунт.

That sounds more marked and emphasizes too dark.

But for a learner, the original sentence is the most straightforward and neutral version.

Grammatically, оказаться is a -ся verb, but in this sentence it does not have a literal reflexive meaning like himself.

You do not need to think of it as the substrate showed itself. It is simply the standard verb оказаться, meaning:

• to turn out
• to prove to be

So it is best learned as a complete dictionary word: оказаться.

In theory, Russian can use short adjectives in some structures, but with оказаться, the normal modern pattern is the instrumental full adjective:

• оказался слишком тёмным

A version like оказался слишком тёмен sounds much more literary or unusual to most learners.

So the sentence you have is the standard, natural way to say it.