More copper is absorbed by the substrate at high pHs, so a release of the absorbed copper occurs when the pH goes down.
Absorbents like carbon and specialty filters and chemicals only reduce copper down to levels safe for most marine fishes. Other marine lifeforms are still killed at the levels of copper concentration left by those 'removers.' So long as the tank remains a FO tank (w/o live rock), there should be no problem with dying non-fishes.
However a shift in pH may release enough copper to kill fishes before those absorbents have a chance to reduce it. Keep in mind those absorbents are not where the rock and those things are that hold/capture the copper.
For the most part, the fish wouldn't die. The poisoning being brief would reduce their defenses, stress the fish, and shorten their lifespan.
