like they said b4,,keep your eye on them....here is what I found on them...
Holothuria hilla is known as the Tiger Tail Sea Cucumber and the Sand Sifting Sea Cucmber. Holothuria impatiens is also known as the Tiger Tail Sea Cucumber. H. hilla has a long, gray to chestnut-brown body with white spots and numerous spiny papillae. H. impatiens has a knobier appearance and is more pale in color. Both of these sea cucumbers are cylindrical in shape and have blunt ends. They can grow up to 6 feet in length in the wild, but most will only reach 18" in the aquarium over time. Due to the seasonal availability of these species, orders will be filled with the species that is available at the time of shipping.
The Tiger Tail Sea Cucumber is one of the few species that attaches its lower body to the inside of its shelter and only extends its anterior half when searching for food. Its meals consist of whatever algae and bacteria, and meaty items are present in the sand that it can scavenge. Due to its feeding habits, it will require a larger tank with a thick sandy bottom and rock caves for shelter. In general, keep only 3 inches of worm for every 20 gallons of water. It is very sensitive to high levels of copper-based medications and will not tolerate high nitrate levels.
It will need to be kept with peaceful tank mates to avoid potential accidents. If it feels overly threatened, it will retract back into its shelter. It does not have tubules of Culvier, but in the wild, if it is attacked or injured, it will expel its internal organs, which may be toxic to fish.
I had an atlantic get sucked in to one of my powerheads a few months ago but they do not release the toxins and I still saw my amonia levels go up for a day or so. I had to take out the powerhead and clean it all on the inside just to make sure it was all out...good luck...let us know what happens...