[somewhatshocked]

Flourite Black Sand? If so, it's ultimately going to get mixed in with the Amazonia over time. Which won't really be a problem - just be prepared. And Amazonia won't break down as quickly as you think it will.

The lower intake on the Flex is there to prevent filter burnout if you allow the water level to drop below the upper intake, so you shouldn't have a problem. You could seal it up entirely if you wanted. Just be sure to do top-offs so you still have water flow into the filter compartment.

 

[somewhatshocked]

There won't be much of a dead zone because of the way the Flex is designed. I have the smaller version and find that the bowed front glass directs water everywhere in the tank. My lower intake is mostly blocked by plants and stainless steel mesh I have siliconed over the grating (to prevent shrimp and smaller substrate particles from getting sucked up) and I haven't had an issue in two years.