A method for filtering liquid in an aquarium, comprising the following steps: - providing a liquid filtration device having a casing comprising a first side, a second side and a water inlet, grid panels (11, 12) inside both sides of the casing, an inner filtration body (41, 42) and an outer filtration body (61, 62) provided respectively inside and outside the grid panels (11, 12), and filtration material (21, 22) between the inner filtration body (41, 42) and each grid panel (11, 12), respectively, - passing the liquid to be filtered through the filtering device by flowing through the water inlet into the filtration device, the clean liquid filtered flowing out from the grid panels (11, 12), wherein the casing of the filtration device includes two enclosures as the sides of the casing with symmetrically set grid panels (11, 12), wherein the two enclosures are integrated as a whole by means of ultrasonic welding or adhesive, characterized in that: - the two outer filtration bodies (61, 62) are respectively fixed outside the grid panels (11, 12) by means of adhesive or heating sticking; - two filtration body hold-down means (51, 52) with grids are arranged symmetrically to a central plane of the casing, the latter being parallel to the grid panels (11, 12) as a symmetrical plane of the filtration device; - the two filtration body hold-down means (51, 52) are fixed respectively inside the casing together with the inner filtration bodies (41, 42), thus forming a first buffer space between the opposite sides of the two filtration body hold-down means (51, 52), a second buffer space between the surrounding of the two filtration body hold-down means (51, 52) and the inner wall of the enclosures, and two third buffer spaces between the corresponding inner filtration bodies (41, 42) and the grid panels (11, 12); - the filtration materials (21, 22) are filled in the two third buffer spaces, respectively. - providing a liquid filtration device having a casing comprising a first side, a second side and a water inlet, grid panels (11, 12) inside both sides of the casing, an inner filtration body (41, 42) and an outer filtration body (61, 62) provided respectively inside and outside the grid panels (11, 12), and filtration material (21, 22) between the inner filtration body (41, 42) and each grid panel (11, 12), respectively, - passing the liquid to be filtered through the filtering device by flowing through the water inlet into the filtration device, the clean liquid filtered flowing out from the grid panels (11, 12), - the two outer filtration bodies (61, 62) are respectively fixed outside the grid panels (11, 12) by means of adhesive or heating sticking; - two filtration body hold-down means (51, 52) with grids are arranged symmetrically to a central plane of the casing, the latter being parallel to the grid panels (11, 12) as a symmetrical plane of the filtration device; - the two filtration body hold-down means (51, 52) are fixed respectively inside the casing together with the inner filtration bodies (41, 42), thus forming a first buffer space between the opposite sides of the two filtration body hold-down means (51, 52), a second buffer space between the surrounding of the two filtration body hold-down means (51, 52) and the inner wall of the enclosures, and two third buffer spaces between the corresponding inner filtration bodies (41, 42) and the grid panels (11, 12); - the filtration materials (21, 22) are filled in the two third buffer spaces, respectively.