An assembly for the formation of a modular tank for containing a fluid, said assembly comprising a first half-shell, and a second half-shell, each half-shell (1; 2) providing a body having a bottom wall and side walls ending in a terminal edge (11; 21) for coupling at a horizontal coupling plane with the terminal edge (21; 11) of the other half-shell (2; 1), said assembly further comprising a gasket that can be interposed between said two half-shells (1, 2) at the terminal edges (11, 21), and fastening means (6, 7, 8) for securing together said half-shells (1, 2) and preventing their movement along a vertical axis (y) perpendicular to the horizontal plane, said assembly being characterised in that each half-shell (1; 2) at the respective terminal edge (11, 21) provides a plurality of protruding external flanges (12; 22), each protruding external flange (12; 22) having a flat surface substantially parallel to the horizontal plane, in that one or more holes are obtained on each of said plurality of protruding external flanges (12; 22) for the passage of said fastening means (6, 7, 8) for securing together said half-shells (1, 2), in that each half-shell (1; 2) further provides a vertical extension (13; 23) for each of said plurality of protruding flanges (12; 22) of the respective half-shell (1; 2), each vertical extension (13; 23) being arranged externally on the terminal edge (11; 21) of the respective half-shell (1; 2) on the respective protruding external flange (12; 22) and projecting along said vertical axis (y) in an opposite direction to the body of the respective half-shell (1; 2)and having the surface entirely flat having a vertical development parallel to said vertical axis (y), in that each half-shell (1; 2) provides a plurality of housing seats, each housing seat being arranged externally on the respective terminal edge (11; 21) and corresponding to each interspace between two adjacent protruding external flanges (12; 22) of the same half-shell (1; 2), each of said interspace having no vertical extensions, and in that said assembly is configured in such a way as to assume an assembled tank configuration by coupling together said two half-shells (1, 2) with their respective terminal edges (11, 21) in correspondence with said horizontal plane, by arranging the vertical extensions (13; 23) of a first half-shell (1; 2) alternately to the vertical extensions (23; 13) of the second half-shell (2; 1) and so as to be projecting towards the mutual half-shell (2; 1) in such a way that each vertical extension (23; 13) of a half-shell (1; 2) fits into the respective interspace between two adjacent protruding external flanges (12; 22) of the mutual half-shell (2; 1) so as to be adjacent and substantially tangent to the side walls of the body of the mutual half-shell (2; 1), by interposing said gasket and by securing together said plurality of protruding external flanges (12; 22) of the two half-shells (1, 2) through said fastening means (6, 7, 8) passing through said one or more holes on each of said plurality of protruding external flanges (12; 22). each half-shell (1; 2) at the respective terminal edge (11, 21) provides a plurality of protruding external flanges (12; 22), each protruding external flange (12; 22) having a flat surface substantially parallel to the horizontal plane, in that one or more holes are obtained on each of said plurality of protruding external flanges (12; 22) for the passage of said fastening means (6, 7, 8) for securing together said half-shells (1, 2), in that each half-shell (1; 2) further provides a vertical extension (13; 23) for each of said plurality of protruding flanges (12; 22) of the respective half-shell (1; 2), each vertical extension (13; 23) being arranged externally on the terminal edge (11; 21) of the respective half-shell (1; 2) on the respective protruding external flange (12; 22) and projecting along said vertical axis (y) in an opposite direction to the body of the respective half-shell (1; 2)and having the surface entirely flat having a vertical development parallel to said vertical axis (y), in that each half-shell (1; 2) provides a plurality of housing seats, each housing seat being arranged externally on the respective terminal edge (11; 21) and corresponding to each interspace between two adjacent protruding external flanges (12; 22) of the same half-shell (1; 2), each of said interspace having no vertical extensions, and in that said assembly is configured in such a way as to assume an assembled tank configuration by coupling together said two half-shells (1, 2) with their respective terminal edges (11, 21) in correspondence with said horizontal plane, by arranging the vertical extensions (13; 23) of a first half-shell (1; 2) alternately to the vertical extensions (23; 13) of the second half-shell (2; 1) and so as to be projecting towards the mutual half-shell (2; 1) in such a way that each vertical extension (23; 13) of a half-shell (1; 2) fits into the respective interspace between two adjacent protruding external flanges (12; 22) of the mutual half-shell (2; 1) so as to be adjacent and substantially tangent to the side walls of the body of the mutual half-shell (2; 1), by interposing said gasket and by securing together said plurality of protruding external flanges (12; 22) of the two half-shells (1, 2) through said fastening means (6, 7, 8) passing through said one or more holes on each of said plurality of protruding external flanges (12; 22).