Given information:
- Mixture contains 30 liters in total
- Ratio of milk to water is currently 2:1

To find:
- Amount of water to be further added to change the ratio to 1:2

Let's solve this problem step by step.

Step 1: Find the current amount of milk and water in the mixture
- Since the ratio of milk to water is 2:1, we can divide the mixture into 3 equal parts.
- Let's assume that the amount of milk is 2x and the amount of water is x. (Where x is a constant)
- Therefore, the total amount of milk in the mixture is 2x liters, and the total amount of water is x liters.
- According to the given information, the total amount of the mixture is 30 liters.
- So, we can write the equation: 2x + x = 30
- Simplifying the equation, we get 3x = 30
- Dividing both sides by 3, we find that x = 10.

Step 2: Calculate the current amount of milk and water in the mixture
- Since x = 10, the current amount of milk in the mixture is 2x = 2 * 10 = 20 liters.
- The current amount of water in the mixture is x = 10 liters.

Step 3: Find the desired amount of milk and water in the mixture
- According to the desired ratio of milk to water, which is 1:2, we can assume that the amount of milk is y and the amount of water is 2y. (Where y is a constant)
- Therefore, the total amount of milk in the mixture should be y liters, and the total amount of water should be 2y liters.

Step 4: Calculate the desired amount of milk and water in the mixture
- Since the total amount of the mixture after adding more water is not given, we can assume it to be 'a'.
- So, the equation for the desired ratio becomes: y / (2y + a) = 1 / 2
- Cross-multiplying the equation, we get: 2y = 2y + a
- Simplifying the equation, we find that a = 0.

Step 5: Calculate the amount of water to be further added
- Since a = 0, it means that no additional water needs to be added to achieve the desired ratio of 1:2.
- Therefore, the amount of water to be further added is 0 liters.

Hence, the correct answer is option C) 30 liters.