I know a few companies produce hang on boxes, I got a decent sized one with an air lift system from amazon a while ago. A series of these with maybe a poret baffle system would let you divide and seperate adults and fry, while also only needing one extra outlet.
If nothing else, check out these boxes, see how they work, flow, etc. Bettas need super low flow as fry from what I have read, too much flow could stress them or dump them into the main tank.
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