Leca as substrate for terrarium -- algae?

[estación seca]

A half inch of water in the bottom would give the same humidity as 2.5". Do you need more than one layer of LECA below the level of your drain? Evaporation and humidity are proportional to the surface area, not the water volume. Having a very shallow LECA layer would give you more growing volume above, which is what every terrarist wishes they had more of. Raising humidity in terraria is generally not a problem.