Abstract: A teaching apparatus or teaching tank which has two, a front and a rear, substantially planar walls at least one of which is transparent. A plurality of bolts and spacers, or molded fasteners are used for holding the front and rear walls in parallel opposed spaced relationship to form the tank volume which tank volume is defined by the planar dimensions of the front and rear walls and the tank cavity width which width is determined by the spacers. The tank side walls and bottom are formed using flexible and deformable tubing, such as PVC tubing which is sealingly compressed by the front and rear walls when they are assembled and defines in combination with the planar walls, at least one working cavity having a working cavity volume. The volume and/or volumes is/are variable and a function of the placement of and the dimensions, the length and the OD, of the tubing and the area of the planar walls.