Sea cucumbers vary a lot in size and the way they feed.
Some species are good at keeping the sand clean while others are filter feeders.
The size can very from 1" (2 cm) to 3' (1 meter).
All sea cucumbers will shrink if they are not provided with enough food.
If a sea cucumber dies or gets extremely stressed it might release toxic chemicals, which might poison fish and invertebrates.
This is rare, but you should ensure that it doesn't get stuck in pumpes or the overflow.
Sea apples pose a greater risk of poisoning the tank than the other sea cucumber species.
This genus of sea cucumbers filters the substrate for food.
These sea cucumbers should only be kept in tanks with a large area of sandy substrate.
They leave behind "spaghetti" like sand, which is a good sign that enough food is available.
It is important that the sand is fine grained (less than 2 mm). Only large specimens will be able to filter 2 mm sand.
A rule-of-thumb says that you can stock 3" (7 cm) of sea cucumber per 20 gal (100 liter).
If the sand is not rich on food for the sea cucumber, then even more space is required.