even an arrow crab? and how about a dwarf sea hare?
Arrow crabs from what I have seen and read aren't culprits of rearranging tanks. However, they're not compatible with some inverts and may go after smaller snails if they get hungry.
EDIT: forgot to add that I've also heard a number of accounts of arrow crabs taking small fish. I haven't been too sure of the validity of these claims, but it probably warrents some stocking caution anyway.
As for the dwarf sea hare, this is another thing that doesn't belong in a brand new tank, but could perhaps be added later if either you have sufficient algae and/or you plan on target feeding it daily. Sea hares eat a LOT and need food every single day to stay healthy. Many species also don't live very long, particularly at higher temperatures (often only 1 year, sometimes 2), particularly if they are allowed to spawn. Also, be absolutely sure you know what genus you're getting, preferably the exact species (although Dolabella auricularia and gigas are may be impossible to tell apart by eye). Dolabella and Bursatella species are more easily kept, but IME Bursatella can be more fragile (although perhaps still easier to keep than Aplysia). Tanks intended for sea hares also need specific safety precautions. In addition to the usual rule of not keeping any fish/crustaceans that will pick at the sea hare, all powerhead intakes MUST be covered with a sponge. Simple plastic mesh, even mesh with holes that look far too small to be risky, will still risk a trapped animal that requires destroying the intake cover to free the animal.
most people avoid nudibranchs (sea slugs & sea hares) because they are hard to feed. Some feed on algae, which is not such a problem in captivivty but they can still starve if the wrong alga isn't available. Many sea hares big and when stressed can release ink into the water. And if a big one dies in the tank it will make a mess of the filtration, (cause ammonia spike). It is preferable to not have nudibranchs in captivivty.
Eh...I don't entirely agree with this for more commonly kept species - which are opisthobranchs, not nudibranchs, as I was once corrected in one of my first ever posts here asking about them. Sea hares are more feasible in most tanks than lettuce sea slugs, simply because they tend to have a wider dietary range and will frequently accept dried seaweeds as well. Although some Aplysia will ink under certain stimuli (the California sea hare being well known for that if the pictures are anything to go by), the more commonly sold Dolabella and Bursatella species are much harder to make ink. If they get seriously injured by a predator/powerhead or if they die of old age/starvation and are left to sit and decay for a week, then you'll see ink. I've never actually heard of a Dolabella species inking just because of stress alone, but rather because of bad injury or quite a long time after death. If caught quickly (e.g. within a day, which is reasonable for catching a dead animal), a dead sea hare will not make the tank into a mess.
/dalibor-andres.from.hr/uw/slike_m/o_012-0_180.jpg ), then IME it's a very peaceful species compared to most of the others that are commonly sold. Some individuals will show a faint light stripe on the red tip but it's not always present. They get easily bullied by other hermit species since they do stay somewhat small and aren't all that strong. They probably shouldn't be trusted with small snails and Nerites, but Turbos would be safe, provided they are bigger than about 1/2" and there are empty shells around. However, I wouldn't stock more just because of the "dwarf" label. It's a strange species where many individuals will stay small, but then one or two individuals in a tank may grow to inhabit shells up to 1.5-2" in length depending on the shape of the shell. In my own tanks, I haven't seen that event to cause any aggression as sometimes occurs in other species when the same size difference happens. Right now my 20g has 4 of that species, and one of the females rapidly grew to twice the size of all the others, but the large one hasn't behaved any differently since the size increase.
