Баланс рахунку низький; я поповню рахунок.

Because the noun is in two different grammatical cases.

• баланс рахунку = the balance of the account
  Here, рахунку is in the genitive case, because it means of the account.
• я поповню рахунок = I will top up the account
  Here, рахунок is the direct object, so it is in the accusative case. For this masculine inanimate noun, the accusative looks the same as the nominative.

So the change is:

• рахунок = account
• рахунку = of the account

Ukrainian often expresses this idea with a noun + genitive structure:

• баланс = balance
• рахунку = of the account

So literally it is balance of the account, which English usually turns into account balance.

This is a very common pattern in Ukrainian. Compare:

• номер телефону = phone number, literally number of the phone
• центр міста = city center, literally center of the city

Because низький has to agree with баланс, the noun it describes.

• баланс is masculine singular
• so the adjective is also masculine singular: низький

If the noun changed, the adjective would change too. For example:

• ціна низька = the price is low
  (ціна is feminine, so низька is feminine)

In your sentence:

• Баланс рахунку низький = The account balance is low

Literally: Balance of the account [is] low.

In the present tense, Ukrainian usually leaves out the verb to be.

So:

• Баланс рахунку низький
  literally looks like: Balance of the account low
• but it means: The account balance is low

This is completely normal in Ukrainian.

If you want past or future, forms of бути appear more clearly:

• Баланс рахунку був низький = The account balance was low
• Баланс рахунку буде низький = The account balance will be low

Поповню means I will top up / I will replenish.

It is the 1st person singular future form of поповнити.

So:

• я поповню рахунок = I will top up the account

This verb is commonly used for adding money to an account, phone balance, card, etc.

Examples:

• поповнити рахунок = to top up an account
• поповнити баланс = to top up the balance

Because поповнити is a perfective verb. In Ukrainian, perfective verbs form a simple future.

So:

• поповню = I will top up

You do not need буду here.

Compare:

• поповнювати = imperfective, to be topping up / to top up regularly
• буду поповнювати = I will be topping up / I will top up repeatedly
• поповнити = perfective, one completed action
• поповню = I will top up

In this sentence, the speaker means one completed action, so поповню is the natural choice.

Not always. Ukrainian often drops subject pronouns when the verb ending already makes the subject clear.

So both are possible:

• Я поповню рахунок = I will top up the account
• Поповню рахунок = I’ll top up the account

Adding я can make the subject clearer or slightly more emphatic. In your sentence, я is perfectly fine.

The semicolon links two closely related independent clauses:

• Баланс рахунку низький;
• я поповню рахунок.

The relationship is easy to understand from context: the balance is low, so the speaker will top it up.

You could also express this with conjunctions, for example:

• Баланс рахунку низький, тому я поповню рахунок.
  = The account balance is low, so I will top up the account.
• Оскільки баланс рахунку низький, я поповню рахунок.
  = Since the account balance is low, I will top up the account.

The semicolon is a punctuation choice that keeps both clauses short and direct.

Yes. Ukrainian word order is fairly flexible, and низький баланс рахунку is grammatically possible.

Compare:

• Баланс рахунку низький = The account balance is low.
  This is a full statement.
• низький баланс рахунку = low account balance
  This sounds more like a noun phrase, not a complete sentence by itself.

So in your sentence, Баланс рахунку низький is the better full-clause version.

Here it means account.

The word рахунок can mean different things depending on context, including:

• account
• bill
• score
• count

But with баланс and поповнити, the meaning is clearly account.

Examples:

• банківський рахунок = bank account
• поточний рахунок = current account
• поповнити рахунок = top up the account

Yes, it is understandable and grammatical. It sounds somewhat formal or system-like, which fits financial or service contexts.

You may also hear more everyday alternatives such as:

• На рахунку мало коштів. Я поповню рахунок.
  = There is little money in the account. I’ll top up the account.
• Баланс низький, я поповню рахунок.
  = The balance is low, I’ll top up the account.

But your original sentence is perfectly valid, especially in technical, banking, or mobile-service language.