Like VaughH said, these are unlikely to be the culprit. However, you could easily kill shrimp (or fish, for that matter) by overdosing excel by a significant amount. I won't venture what that amount is cause it's not an experiment I've done myself, but it's much, much closer to the recommended Excel dose than the lethal doses of CSM+B or KNO3 are to their respective recommended doses. To rephrase that in pharmacological terms, the "therapeutic index" of Excel is narrow.