Catchment costs?

[MarkP]

I believe the goal is to introduce the water from the roof, which is considered dirty, to the bottom of the tank using a diffuser such that there is as slow and smooth an upward flow as possible. This results in settling out of anything that will sink. Consequently the water at the top is freer of settleable material than the water at the bottom is. This is only true if the water is introduced gently. You don't want "circulation" or for the water to leave the downspout with any appreciable velocity. It only winds up going up because there is no more room at the bottom, not because it got fired in that direction. Various diffuser designs attempt to achieve this effect. If it s working right there will be a layer of dirt on the bottom. If that dirt gets scoured up during a rainstorm you have not achieved the desired effect.

Why is this good? Only because you have a floating suction intake that draws water from just below the surface. It's a package deal.