The two red Xs that appear on slide 12, in particular, just mean that those two factors weren't analyzed in that case, because with flow-through and no aeration, the amount of water required for the stocking densities for the flow-through was unfeasible. I can't remember the numbers offhand, but they were unattainable. It was not possible to pump that volume of water through that size.
On October 27th, 2011. See this statement in context.