*Changes*
The giants believed that the poison killed all the Fishmen, but quite the contrary;
the Fishmen were laying low, devising an elaborate plan to use the catalyzed poison to taint the giants' drinking water.
They targeted the revered Stone Mallet Prophets, believing that eliminating them might deter other giants.
*Once the quest is complete, read this book to report back.
(You wondered about the details of this plan.)
The Fishmen managed to sneak the poison into the giants' water, thus killing the Prophets. The giants were stunned by this turn of events.
The Fishmen charged the giants in the middle of a hot, sunny day. With no drinking water left, the giants had no choice but to retreat. The Fishmen were triumphant!