“My wages are being garnished but they’re taking after tax income. Does that mean that I can deduct from my income what’s being seized by the garnishing creditor?”

Unfortunately, the answer to that questions is no. Because it’s after tax income, that income is net of what you already had deducted from your paycheck. So, you’ve already paid taxes on your salary or your wages and now the garnishment is coming out of your after tax income. That’s one of the reasons why garnished wages are so debilitating and so difficult to overcome.