What about a square, divided into thirds using opaque barriers, with the two outside thirds as living quarters and the middle as your trial space? Then you could create a flow-through or recirc system with the water easily moving throughout the setup by positioning two powerheads in the trial section (assuming your movable doors from each living space were permeable - some kind of grating (not too fine of course) would be ideal), where the octos can't normally get to them to get into trouble. See attached rough schematic, with boxes for powerheads and arrows indicating flow. You could remove the powerheads during use of the middle section. You would only have to worry about positioning the powerheads out of reach through the grate...
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