Stay with the holothurm species. the blacks are thin skinned and damage easily, once introduced with out injury they are fine. the tiger tail are ok, i like the brown ones that look like the black. but have not seen them for a while. (they look like a turd).
your tank is not ready for a cucumber. they injest the substrait and grind it in their intestines to remove bacteria and other particulate and then poop out a clean product at the other end. it comes out in a roll and breaks back down and mixes with the substrait quickly. you need a well established sand bed and substrait that is not sharp. also the substrait needs to be reef select or fine grade. hope this helps.
