Congrats on the awesome tank man!! I thought I was the first to think about over hanging branches for silver arros to pick off their prey but obviously not. Archers would be cool for that to, some food for thought maybe.
I was thinking about the cover and came up with an idea that may or may not be a good one. Instead of using something as solid as glass etc as a hood, you could use like a thick plastic sheeting material like the liner used for ponds but of course is transparent, that way it has some give and won't hurt the fish but should be strong enough to contain them.
I know this is a bit late but one way to measure how much water is in your tank is to get a container that you know the volume of. Example a 10gallon drum. than fill up the drum and time how long it takes to fill. E.g. say it takes 1 minute. Than using the same flow rate time how long it takes to fill the tank. Say it takes 100 minutes. 100minutesx10gallons= 1000gallons.
Not saying it will be that simple but you get the idea. May need to break it down to seconds etc.
Anyway you are giving me motivation to get my plywood tank started, which will be near exact dimensions.
My buddy is a landscaper with a gov contract on the Yough reservoir, and they were cleaning up driftwood at the dam. The burned 10 tons of this stuff 
I told them I would've taken it and sold it. Oh well. Theres other lakes around.
It's gonna be one big stinkin rock!
