Goldfish:
-Lives in a cold water environment.
-Does best with a neutral ph, not base or acidic water.
-Loves water, no land required.
Crab:
-Lives in a tropical environment of at least 75 degrees.
-Lives in brackish (basic) water. Needs salt in the water to survive longer than a week or two.
-Requires access to land to survive, not purely water.
The crab will live a week, two tops. It will probably be harassed by the fish as well, quickening it's death. May also attack fish, leaving open wounds on your goldfish which will soon lead to infection.
WHAT AN AWESOME IDEA OP DURRHURR :D
If you wanna kill some animals for enjoyment, why not throw an insect in your fish tank and watch your goldfish devour them? More entertaining, less expensive and damaging...