I would say you're right, low DO they were drowning. I don't think you need to flush so much as lower the water level to at least an inch or two below the net pot and get some bubbles in there -- as much as you can. I think you should have minimum 2 airstones per plant. I think that's about all you can do...I think they will recover, but I don't really know cause that's never happened to me before.
