______ is used as a decolourizing agent in sugar refining industry and to filter aquarium water.

Explanation

**Step-by-Step Solution:**

1. **Identify the Context**: The question asks for a substance that is used as a decolourizing agent in the sugar refining industry and for filtering aquarium water.

2. **Consider Common Substances Used**: In the sugar refining process, certain materials are known to be effective in removing color from sugar solutions. One of the common substances used for this purpose is a form of charcoal.

3. **Specify the Type of Charcoal**: There are different types of charcoal, but the one specifically mentioned in the context of sugar refining and filtering is "bone charcoal."

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