If you want to increase the airflow, one fan pushing and one pulling is going to be largely redundant. Just leave an opening where the 2nd fan (pulling out of the cabinet) would have been, air forced into the cabinet will exit there and through the plumbing holes. If you really want to add a 2nd fan, I would not recommend that you use it to pull air out of the cabinet as it will then be breathing moist salty air and will shorten the lifespan of the fan. Always push dry fresh air if possible. Of course if you add a 2nd fan, be sure to have enough holes in the cabinet to allow for an equal amount of air to exit as you are pushing in through the fans. That being said, a single 120mm fan will move about 30 cubic feet per minute which would completely change the air in a 120gal tank's cabinet aprox. 80 times in an hour, which is a lot.
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