You want what they call 6mm hexmesh it has small mesh and keeps all but the finest out we used to use it but the net wore out after 6 seasons .
We didnt bother replacing it this time around after loosing the paranoia after we had a jumper our first season out doors ,it also had a bungee strap back bone which alowed it to be pulled exta tight on two kleats the circumference of the net was also similarily bungee straped that allowed you to tighten up around the circumference of the pond allowing no preditors in , so it had nummerous uses to us .
1) it kept preditors like herons at bay being such a fine mesh.
2) it kept all but determind preditors out.
3) it kept our fish in the pond by acting as a high strung safety net.
4) it kept leaves off the ponds surface .
4) it could be left in place during the winter under the policarbonate roofing sheets .
Dave