To provide an ecological apparatus using safe natural materials, wherein a water contamination preventive device is set at an optional place in a conventional fresh fish water tank, contaminants like water scale rapidly generated by feeding oxygen to the water tank are solidified and precipitated by magnetism, the precipitated solid matter is hardly dissolved into water and easy to remove, thereby preventing turbidity of water in the fresh fish tank, transparent and clean water can be always maintained, promoting oxygen feed effect to water, the frequency of replacement of water of the water tank can be greatly changed, and preservation of fresh fish and hygiene can be excellently kept.

The water contamination preventive device for a pure-dora water tank solidifies fine particulate excrement causing contamination by fresh fish by the reaction of magnetism, forms a hardly dissolvable state and turns to easily removable matter, to keep freshness of fish.