how many cukes?

[30 hex]

30 hexagon with 20 lbs live sand. should i get 1 ir 2 cucumbers?

 

[mantis]

You should not get any, when they die or are attacked by another fish, they can release a powerful toxin that can kill tank inhabitants.

 

[burnnspy]

There has been very little research onto which cukes are toxic, I recommend avoiding them also.
BurnNSpy

 

[es53]

The smaller cukes are generally considered safe, or at least safer, such as the Tiger Tail Cuke. Julian Sprung & Charles Delbeek recommend them for keeping the sand clean in The Reef Aquarium Volume 1. They say to avoid large cukes (over 6 inches) and never to get a "Medusa Worm".