It is one of the most beneficial _____  _______ _________ because the water containing fish waste is used to fertilize the agricultural field.

Sure, here is a step-by-step breakdown of the answer:

Step 1: Identify the missing words in the sentence. The sentence is asking for the most beneficial something that is used to fertilize agricultural fields.

Step 2: Think about the context of the sentence. It mentions that water containing fish waste is used for this purpose.

Step 3: Based on the context, we can infer that the missing words are related to the practice of using fish waste to fertilize agricultural fields.

Step 4: Fill in the missing words. The most beneficial practice that uses water containing fish waste to fertilize agricultural fields is called aquaponics.

Step 5: Put it all together. The completed sentence is: "It is one of the most beneficial aquaponics practices because the water containing fish waste is used to fertilize the agricultural field."