That said, this actually sounds like a pretty textbook case of "cuke nuking" - meaning your cucumber dying and poisoning the tank is almost guaranteed to be the cause. The inverts are likely okay because the toxins weren't present in high enough quantities to kill them. Sea cucumber toxins (called holothurin) are ichthyotoxins, meaning they are mainly toxic to fish. In high enough quantities these toxins will kill inverts and if touched or ingested can cause problems for people too, but they primarily affect fish. Holothurin causes irreversible damage to a fish's gills and nervous system (amongst other things) - this would explain the discolored and puffed out gills you described.
