I have a 40-gallon aquarium (36x13x20) which will be planted. I bought 2
7kg bags of Fluorite, as this is the amount recommended on the bags.
That's roughly 30 pounds. If I would be using roughly 50-60 pounds of
gravel total, should I count the Fluorite as gravel? I will be using a
substrate heater which I plan to pack in a layer of small-pebble
Conaroc. I suspect that layer itself will be in the neighborhood of 25
pounds. Then the (rinsed) Fluorite, and we're already at 55 pounds. I
guess I shouldn't plan for a very thick layer of gravel on top of the
Fluorite? I had thought to make a "sandwich". Tips appreciated.

Adam Gottschalk wrote:

I have a 40-gallon aquarium (36x13x20) which will be planted. I bought 2
7kg bags of Fluorite, as this is the amount recommended on the bags.
That's roughly 30 pounds. If I would be using roughly 50-60 pounds of
gravel total, should I count the Fluorite as gravel? I will be using a
substrate heater which I plan to pack in a layer of small-pebble
Conaroc. I suspect that layer itself will be in the neighborhood of 25
pounds. Then the (rinsed) Fluorite, and we're already at 55 pounds. I
guess I shouldn't plan for a very thick layer of gravel on top of the
Fluorite? I had thought to make a "sandwich". Tips appreciated.

Sandwiches come apart, particularly when the grains are different sizes.
Hopefully all your gravel is smaller grain size than the Flourite, as gravel
larger than that is not useful for plants. Use at least 50% flourite by
volume, or skip it as you will lose the benefit.