with a sieve passing fraction at a mesh width of 250 μm of more than 75 % by weight and at a mesh width of 500 μm of more than 99 % by weight, and

It has a mesh width between 25 and 80 μm.

with a sieve passing fraction at a mesh width of 250 μm of more than 75 % by weight and at a mesh width of 500 μm of more than 99 % by weight, and

a sieve passing fraction at a mesh width of 250 μm of more than 75 % by weight and at a mesh width of 500 μm of more than 99 % by weight, and

The second fabric layer (3) has a lesser porosity or mesh width than the first fabric layer (2').

a sieve non-passing fraction at a mesh width of 120 μm of not more than 1 % by weight

a sieve non-passing fraction at a mesh width of 120 μm of not more than 1 % by weight

a sieve non-passing fraction at a mesh width of 120 μm of not more than 1 % by weight

a sieve non-passing fraction at a mesh width of 120 μm of not more than 1 % by weight

a sieve passing fraction at a mesh width of 500 μm of more than 99 % by weight and

a sieve non-passing fraction at a mesh width of 120 μm of not more than 1 % by weight

a sieve passing fraction at a mesh width of 500 μm of more than 99 % by weight, and

a sieve passing fraction at a mesh width of 500 μm of more than 99 % by weight, and

The wire mesh is characterized by the wire diameter D, the mesh width B and the mesh length L of the mesh, the ratio L/D being in the range from 25 to 60, boundary values inclusive.

The bottom of the breeding tank is closed off by a filter (4) or filter-like device of suitable mesh size.