The answer is given by 15 choices for the first fish, 14 for the second, 13 for the third, up to 4 for the last fish. That's 15*14*13*12*...*5*4=217,945,728,000. But if the order doesn't matter, just the combination, then we reduce this by the number of ways 12 different fish can be arranged (479,001,600) giving 455.