Originally Posted by Tihsho
With the slope being at 45 degrees you can effectively deduce that the volume of capable water would be 50% of the math I provided. As for drains, on any project, I always recommend running a double drain. This will provide you a backup drain if something clogs. Also, you can run the second drain at an alternate height, so if you get a small restriction that raises the water, you won't have to worry about an overflow if the secondary drain is above the primary.
I don't think I've done a good job explaining the situation. Let me use a different example. Lets say you have a pint glass. It holds exactly 16 oz of water. Now fill that pint glass to the brim with fine gravel. How much water does it hold?
A lot less then 16 oz that is for certain. The easiest way to find out would be to pour water in till it reaches the brim. This is the scenario I have in my tank. My substrate and false bottom will be homogeneous. So I won't be able to calculate how much water my tank holds without actually filling it up. It won't be much though.
One reason to use a canister filter over a sump though is that a canister filter doesn't rely on gravity to drain water. A canister filter can suck the water out of the tank. This will be useful for me because my bulkhead due to practical build considerations can not be at the lowest point on the viv. I wouldn't have any room to install it there. Instead it will be a little higher up and I can run a tube down to the lowest point in the viv and thus be that much more protected against running dry. Hopefully I will be able to dial in my humidity and misting to such a degree that I will be able to ensure my canister never runs dry but I won't know for sure on that point until after its setup and running for a bit.