I have seen a couple of very healthy tanks lose their fish compliment mere days after the addition of sea cucumbers...after years without a hitch. Both tanks contained Centropyge spp. angels, but I am less than sure that they stressed out the cukes...one tank had definite evidence of evisceration. Neither tanks had powerheads in them, but rather large overflows which neither cucumbers were near when I arrived...
One of the cukes was a pink Holothuria edulis?
On another note...Cucumbers can die in your system with no apparent ill effects because the Holothurin toxin is almost exclusively contained in the tissues and organs. Cucumbers do not release a cloud of soluble toxin into the water. This defense mechanism would be useless on an actual reef...The toxic effects come from fish consuming tissues, tubules, gametes or ground up pieces of organs.
As far as my experience Cukes don't eat hair algae.
an in-depth discussion of cukes:
http://www.advancedaquarist.com/issu...003/invert.htm
Stephen