Tank apparatus with movable rollers

An apparatus comprising (i) a tank having a base and sidewall enclosure extending from the base suitable for containing water, (ii) at least one set of three rollers suitable for being positioned within the tank, wherein each roller extends longitudinally between roller ends and wherein one roller of each set has a density of less than a density of water so that the roller is floatable in water, and (iii) guides configured and positioned within the tank for guiding and positioning the rollers (a) so that, with respect to the tank being positioned so that the base extends horizontally and the sidewall extends vertically, the rollers of the set (1) have parallel longitudinal axes which extend horizontally and (2) are movable vertically and (b) so that, with respect to a horizontal plane, the floatable roller is positioned between and spaced at a distance from the two other rollers of the set and wherein:

• the guides which guide and position the floatable roller are configured and positioned within the tank to control vertical movement of the floatable roller so that, upon the tank containing water, the floatable roller is positioned beneath a surface of the water;
• and
• the guides which guide and position the two other rollers of the set are configured and extend a distance for moving each such roller vertically to a position above and to a position below the position of the floatable roller.