Leviticus 11:10
And all that have not fins and scales in the seas, and in the rivers, of all that move in the waters, and of any living thing which is in the waters, they shall be an abomination unto you: And all that have not fins and scales in the seas, and in the rivers, of all that move in the waters, and of any living thing which is in the waters, they shall be an abomination unto you:
Every creature that moves in the seas and in the rivers and lacks both fins and scales is declared an abomination and unclean, and is therefore forbidden as food. It belongs to the requirements given through Moses that forbade eating any unclean thing, including swine's flesh and the flesh of certain other animals, while the animals permitted were those least objectionable for food. It was given as a health principle and sanitary law during strict training in the wilderness to cleanse the people from the unclean and unhealthful habits learned in slavery before entering Canaan, and obedience to it brings health. It forms part of the restrictions on fish given to Israel when they settled in Canaan.
The restrictions were intended to lessen the harmful results of eating animal food, because the forbidden creatures are not the best articles of food and are hurtful to health. The prohibition protects health by preserving pure blood and the faculties of mind and body, since such foods fill the blood with impurities and shorten life. If Israel had manifested submission to these prohibitions, God would have removed from them everything injurious to health and would have suffered no sickness to be among them.
Like the creatures without fins and scales, swine's flesh was never designed to be eaten under any circumstances because it fills the system with humors and in a warm climate often produced leprosy with far more injurious influence than in a colder climate, though swine were useful as scavengers left to run free in a fruitful country to devour decaying substances upon the ground that would poison the atmosphere. The meats appointed for Daniel and his companions from the king's table included swine's flesh and other meats prohibited by the law of Moses, and Daniel refused them because eating them would defile him and would leave his mind and body unfit for God's service. The dietary rules were not arbitrary, and they were not to be extended beyond what God required.
The verse does not require filtering or straining all water used lest the smallest speck or insect be swallowed as an unclean animal. That excessive rule about tiny insects was an unwarranted addition that burdened people, made minor matters of external cleanliness seem weightier than serious sins, and strained at a gnat while swallowing a camel.