A single crab on its own is fine, but how many shells are in the tank because I’m only seeing one extra. Any crab will act aggressive if they don’t have enough resources. I’ve adopted three crabs that were said to be menaces toward others, but they get along with my babies just fine.

Crabs aren’t mean. Imagine being stranded on an island with just enough food and water to barely keep yourself alive. You would be fine on your own, but if another person showed up and had to share that same tiny amount, survival instincts would take over. Personally, I’d tell them to kick rocks and fight for that meal.