If you are considering a cucumber, staty away from the black ones, and only use the holothurums, The cucumbers can expel their insides if attacked or injured, there is also a white sticky stuff like unhardened glue under water but stringy that some of um eject also. If you stay away from the black ones, you should be ok. you can find yellow, brown, sometimes pink, but, which ever one you choose make sure it is not too large, the substrait is of the correct size and consistency, and acclimate for at least 2 hours, when you introduce it to the tank do not handle, use a ladle or something to put it in the tank, do no let it get stung by any thing and lower it into an area of low flow. also make sure the one you get is a sand sifter. good luck, I use three in my main tank and one in my refugium.