Fish tank filters are essential equipment. Of course, depending on the placement position, filters can be divided into external filters, top filters, and bottom filters. In terms of specific selection, it needs to decide what kind of filter to choose according to the configuration of the Aquarium and the situation of fish farming. The filters outside the Aquarium refer to the closed and open filters with water capacity installed outside the Aquarium. Mainly used in small and medium-sized aquariums.

External filters are generally shaped like water tanks, containing substances such as activated carbon and filter cotton that can regulate water quality. The water pump is located at the top of the water tank. Through the pipe inserted into the Aquarium, the water is pumped into the water tank from the inlet at the bottom of the pool, and then returns to the water tank after passing through the filter material. In order to start effectively, the external filter should be suspended below the water surface outside the Aquarium, but under no circumstances should the external filter be submerged in water for use. This device has high filtration efficiency, simple maintenance, low noise, and does not occupy too much water space. The chemical properties of the water can be adjusted by selecting filter materials. Therefore, it is a high-end supporting equipment for larger aquariums, especially suitable for feeding cyprinidae and South American snapper Tropical fish that like clean and soft water environment, and is the preferred filter for aquariums to plant Aquatic plant. At present, imported external filters have good quality and high prices. Although the price of some domestic brands is low, the quality of pumps is often substandard, and some are replaced by Submersible pump, affecting the use effect. 

The working principle of the enclosed filter outside the box is that the water inside the box automatically flows into the filter outside the box through a water balance pipe, and then the filtered water is re injected into the box by a mechanical pump to complete the water circulation filtration. 

The open filter body outside the box is a rectangular biochemical filter box. The water in the Aquarium is injected into the filter box, passes through the filter material, and then returns to the Aquarium through the mechanical pump in the filter box. The biochemical filter box is a rectangular three-dimensional box that is 1.5 meters long, 1 meter wide, and 1.3 meters high. Its material is non-toxic polyethylene plastic, with a thickness of generally over 2 centimeters and a solid and firm texture. According to the design dimensions, cut the plastic plate into shape and make the four walls of the filter box. Then, at a high temperature of 200 degrees generated by the welding gun, the plastic plate is bonded together by plastic covered electrodes, forming a rectangular filter box. The interior of the filter box is further divided into several chambers, in which several different types of filter materials are placed.

The biochemical filter box is divided into a coarse filtration chamber, a fine filtration chamber, a biochemical filter material filtration chamber, and a water storage chamber. Among them, the filter materials mainly filled in the coarse filter chamber include acrylic cotton, sponge, artificial fiber, artificial seaweed, etc. Its function is to filter out large particles in water. The filter materials placed in the fine filtration room mainly include gravel, coral stone, and activated carbon, which are used to filter out small particles and metal ions in the water. The biochemical filter room is mainly equipped with a water drop, which is used to absorb beneficial bacteria Nitrifying bacteria in the water, and use Nitrifying bacteria to convert harmful oxygen, nitrogen compounds, nitrate and nitrite in the water into harmless nitrogen, so as to achieve deep purification of water quality. The water chamber stores the filtered water, and then pumps it out with a mechanical pump and returns it to the Aquarium, which is the process of circulating filtration of water quality. Because the water in the Aquarium is filtered layer by layer in the biochemical filter box, its water quality is clearer, clearer, more transparent and can be stored for longer.