I had one once a long time ago. My main advice would be whatever you do make sure that you have a secure lid! To give you an idea, mine climbed up a cable and escaped despite the grid, ran through my entire house and disappeared. We recovered him six months later under the radiator in my bathroom In my experience they're okay with fish, although I would be careful as the ones I had were quite large and could handle themselves, a BN pleco for instance might not enjoy the company. They might also cause a fair bit of damage to your plants if you have any. Whilst they can survive in freshwater, it's a bit like bettas in a bowl, they really need brackish water. They are amusing little critters to watch though but I'd think carefully about getting one.
Do they need to get outof water and with fish how would I do that And last question can I put it in a half gallon for a few days and how much salt should I add for brackish water?
In answer to your first question, I'd give them a way out of the water: rocks, a branch, etc... They need access to land. Concerning tank mates, obviously they have to be able to survive in brackish water. I'd avoid anything with long, flowing fins that the crab could grab a hold of. Pufferfish might give your crab a hard time, I can't say for sure. Personally, I would go with smaller and faster fish who don't hog the bottom of the tank. Perhaps some mollies (not sailfins) or regular guppies? I'd be careful, crabs are carnivores and it's a fair bet that if they can catch your fish at best they'd give them a hard time. Maybe some shrimp might do better. Define a "few days". If it can be avoided I wouldn't keep one in that small a tank for very long but I suppose if it was literally a couple of days and you carried out frequent water changes it might be okay. As far as salt goes, you're aiming for 1.005. What size is your tank?
10 gallon but first in a half gallon so 1 teaspoon of salt for the half gallon or is that to much
For these crabs the ratio is about one teaspoon of salt per-gallon (give or take a bit). So, I'd say about half a teaspoon then, probably a bit less. As I said, make sure it has a good lid. If it can escape, it will.