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A small nano-sized power head would help the water to circulate. Assuming you plan a complete roughly circular path around the tanks, water being pushed into one tunnel would force the water in the whole setup to move. Decorations would help keep the water churning (so to speak) so parts won't get stagnant. Just keep the pump well under the water level to prevent microbubbles from flowing through and getting caught in the different sections.
In this tank setup, bettas would be the worst possible choice. They could get trapped in the lower tanks and not be able to get to oxygen and suffocate.
Are you planning on this being one level of connected tanks? Or multiple tiers?
Eagerly anticipating an update. I've always loved these tanks and thought they must be great for fish as they could really move around. I think I would go for fewer tanks that are larger though, might be easier to clean, easier to manufacture and easier to control ( and probably better for the fish too).