Tried some searches to no avail... how does one keep different substrates separated in an aquascape? I am thinking that there will need to be some type of barrier, between the different type of substrates to keep them from mixing, but i cant figure out what would be a good material. cardboard would bend to different shapes, and not be so wide as to be very visible, especially once plants have grown in, but I think it would turn to mush very quickly and no longer keep the substrate from mixing. any ideas? please? thankx.