When you guys have built your raft tank do you build them level and just the drain on the bottom creates the flow or do you pitch them slightly towards the drain to help water flow. I am building a tank 4'x 10' and plan on water in from the top and water out the bottom on the opposite side with a ball valve to control water flow.
A lot of us will put a drain in the bottom or bottom/side of the end of the trough and insert a "stand pipe" into the fitting to set the water level. The trough can be perfectly level.
You don't want a ball valve because it will foul and restrict flow over time. There is no need to regulate the flow out of the tank.The water will drain at the same speed you pump the water in. If you shut the pump off, the water will stay at the level set by the stand pipe. :)
You're talking about the DWC grow bed, right? A slight (1"?) fall over the length of the trough would come in handy if you ever need to empty the entire thing, but remember, water is going to be level whether or not the trough is.