The other question I came up with is why bother? How many mature tanks have a problem with high nitrates/nitirites? That would be the main advantage of this sort of system, nitrate/nitrite reduction.

I do like the idea of active flow through a gravel bed though. That seems like it would have good biological activity in a small area. Again, this would be along the same lines of a wet/dry without the oxygenating effect of "trickling". Of course there is no reason why you couldn't replace bioballs with LR rubble in a wet/dry.

If I could ever figure out how to do it effectively, I would like to figure out how to build a tank with a spreader bar running through the rockscape of my main tank. I've done it before, but the problem was that the openings in the spreader plugged off with debris and scale build up. It was impossible to remove the bar for maintenancee without tearing apart the rockscape.

Gary