Peel shrimp. Place shells in a medium saucepan. Add 2-1/2 cups water, cover and bring to a simmer over medium heat. Cook 10 minutes.
Meanwhile, heat oil in a large pot over medium heat. Add shallots and sweet red pepper. Cook 5 minutes to soften. Stir in potatoes and cook 1 minute. Strain clam juice into pot, reserving clams. Strain shrimp broth into pot; discard shrimp shells. Bring to a simmer, cook 10 minutes.
While potato mixture cooks, cut shrimp and tilapia into bite-size pieces; whisk flour and salt into milk.
Stir shrimp, tilapia, milk mixture, condensed soup, clams and corn into pot. Simmer 3 to 4 minutes, until shrimp is pink and fish is opaque. Top with crackers and chives, if desired.