The second tank is 48 / 2 = <<48/2=24>>24 gallons.
Following her rule, Gail keeps 24 / 2 = <<24/2=12>>12 two-inch fish in the second tank.
She keeps 48 / 3 = <<48/3=16>>16 fish in the first tank.
If one fish in the first tank ate another, she would have 16 - 1 = <<16-1=15>>15 fish in the first tank.
Thus, Gail would have 15 - 12 = <<15-12=3>>3 more fish in the first tank.
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