I've taken the ACT twice, and I'm worried because my composite score decreased by 5 points on my second attempt. Could this have a negative impact on my application? Or will colleges largely ignore the second score and just look at the first one?
Your score on the second test shouldn't have a negative impact on your application. Admissions officers will assume that you had a bad day and focus on your higher scores. If you choose to take the ACT again, however, you should make sure that you prepare very thoroughly to avoid a lower score. A third score without improvement could lead colleges to question your study habits.
I'd recommend reading this article on why your ACT score may have gone down and what you can do to reverse this trend before the next test date. Consider whether there was a problem with your overall testing strategy or with a particular section or sections. Above all, continue to take practice tests to monitor your score levels as you get closer to your next retake.