i dont know if i asked this before but i would like to add a cucmber for sifting. and sand maintenance. i am wondering if they can climb up tank walls? i dont have any mean fish so powerheads would be my only concern until i add a closed loop system in the next few months

if they die could they kill everything in the tank. this sounds untrue from the magnitude of what is being suggested.

it depends on the particular cuke. tiger tails are one of the species least likely to nuke your tank. tiger tails DO get large, however (12-18") and you need about 20 gals for every 3" of cuke and a lot of open substrate for them to feed from.

thats what i hear. i was gonna keep the cuke and my sand sifting starfish. i figured there would be enough food for the two of them in a 180

generally any cuke from the Holothuria species is safe for the tank. the only threat to wiping out the tank is from being sucked into a powerhead. the flesh of the cuke is toxic, so anything the eats the cukes flesh will probably die. that said i have had one in my 75g. it lived for about 5-6 months then something caused it to spit its guts out. this is their defense method, and sometimes they recover, more often they dont in our tanks. mine didnt make it. after about 2 weeks i presumed he was dead as he hadnt moved from on spot in 2 days, and wasnt looking very good. nothing else died in the tank. heres some more info on cukes.
Aquarium Invertebrates (http://www.advancedaquarist.com/issues/jan2003/invert.htm)

Are there smaller species that are fairly "nuke" safe?

the article discusses how problematic it is to give generally safe cuke recommendations