
Essentially all you need is some floating objects for the water scorpion to grip on, ans also so it can breathe through that long tube on the end of it's abdomen. You can offer one insect a week(watching them eat is AWESOME...it involves injecting a saliva that turns the insides to goo and sucks them out!) although I suggest removing all the legs of the prey item before offering it, otherwise it will twitch as it's being devoured, attracting other fish who will try and steal the water scorpion's dinner. And if you want to vary the diet, you can toss in the occassional fish(it has to be REALLY tiny though) once a month. Their stick-like appearance also helps them stay hidden from fish who may want to snack on it.
The reason I posted this is because I'm watching mine swim around right now...they're really cool.