Bring a pot of salted water to boil and cook spaghetti according to package directions (about 9 minutes); drain and reserve 1 cup pasta water.
While pasta cooks, heat 2 tablespoons olive oil in a large skillet over medium heat, sauté minced garlic for 1 minute, add drained tuna, tomatoes, and olives, and simmer 5 minutes.
Slice ciabatta, brush with 2 tablespoons olive oil mixed with minced garlic, and toast in a 400°F oven for 4 minutes until golden.
Toss pasta with tuna sauce and 1/2 cup pasta water (add more if needed for sauce consistency), divide among four bowls, top with fresh arugula, and serve with garlic bread on the side.