The present invention relates to a tank assembly for making fiber products from stock comprising a stock vat, a main tank (2, 103, 202), at least one inlet (20, 21, 111, 216) to said main tank from said stock vat, and an outlet (22, 113, 217) from said main tank. The tank assembly further comprises at least one mold tank (3, 102, 203, 204) having at least one inlet (37, 114, 218, 219) at its bottom connected to the inlet to said main tank and an outlet (4, 116, 221, 223) connected to said main tank so that stock from the mold tank is arranged to flow into said main tank and in that the horizontal cross-sectional area at the top of said mold tank is larger than the horizontal cross-sectional area at the bottom of said mold tank.