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Definitely the first from a biological standpoint.
Gills work by having water pass through them saturated with dissolved oxygen. The gills have a ton of surface area that pull the oxygen from the water and distribute it to the body. To orient take in the smoke, it works have to enter the mouth first then pass through the gills. The other way the particulates would flow out of the cig and behind the fish
Maybe this fish swims backwards, didn't think about that did you