I use the black expanding foam that is made for ponds, and 'mortar' together some rocks. This makes a divider that looks more natural than the plastic strips, and works just perfectly because the foam sticks just fine to glass and to rocks.
Any 'sort of' divider like a row of rocks with no 'mortar' is going to allow the substrates to mix. I just took down a tank like that. Pool filter sand and Soil Master Select. Totally mixed because the row of rock were just that, nothing between the rocks.