I've been planning the solar greenhouse that I would like to build and want to incorporate some water barrels along the back wall for some thermal mass. But I also would hate to lose the space for growing plants, so I came up with this idea.
A row of 55 gallon barrels with the top 10 or 12" used for a grow bed and a couple of feet of water under. The growing media would be supported by some kind of screen on pvc legs and they would flood and drain in the top section only. I could set them up in groups of four all plumbed together at the bottom and just below the screen.
They would drain to their own sump and the more standard grow beds across the front of the green house would go to another sump.
Two pumps to a constant height fish tank, then through a single set of filters and then diverted to the two systems.

Maybe this has been done before and failed, I don't know. I've got all winter to plan so any input is welcome.

I suck at Google Sketchup so here is a scanned drawing to show what I'm talking about. I'm working on the layout of the whole system and the greenhouse design, I'll post more of that when I get it like I want.