Seriously, I would just slow down and look at your empty tank for now. You cannot "just throw" anything into a cycling saltwater tank, snails and crabs included. Inverts are some of the hardest critters to keep alive if your balances are out of whack.
Your tank needs time to cycle. You need to finish with aquascaping. One bag of sand is probably not enough for a healthy system. After you have set it all up, let it cycle, and had good measurements for a week or two, you can begin to add things, very slowly, and usually one at a time. If I'm following the thread, you have "moved" your SW into a 20g?
You are going to want to stick with smaller fish. You have to remember that with very few exceptions, everything you add to a SW system was very recently swimming in the huge ocean. Putting it into a 20g is going to be a culture shock to say the least.
Take some time, let things settle, do some homework on some small, compatible fish, and plan out what you want. Anything that eats will place a bioload on your system, even snails and crabs.
SW tanks are NOT like FW tanks, in that you can just up and move them around in an afternoon. Any disturbance is likely to set off some sort of cycle. And BTW, even a 55 or 75g may get the tang police after you.