Just a fyi, I'm not able to test it in my environment currently but it has something to do with the large concentration of underwater mobs and likely a bug (or extreme inefficiency) in the underwater checking for aggro code and/or the underwater pathing code. I will eventually look into this and maybe restore the piranhas, but not a high priority since they don't really serve much of a purpose.
Also note that you still need to kill the first 2 enchanters. I haven't checked if you can sneak by them or not.