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Lots of us use powerheads. Adult octos have been known to take them apart! I
The intake needs to be securely covered, so find a good model that has a fine plastic grid (mine does). For a very young octo, I'd probably cover the outflow loosly with something like the end of a carbon bag, something that allows good flow of water.
My bimac liked to place herself over the outflow and "fill up" with water!