Rinse tilapia fillets and dry with paper towels. Dissolve tamarind paste in hot water in a bowl; stir ginger, garlic, salt, black pepper, coriander, turmeric, and cumin into tamarind mixture to make a paste. Rub paste over fish and let stand 15 minutes to absorb flavors (or longer in refrigerator if desired).
Heat vegetable oil in a large nonstick skillet over medium heat. Cook fish with tamarind paste in the hot oil until the flesh is opaque and flaky, about 4 minutes on the first side and 2 minutes on the second side. Drizzle with lemon juice.
This dish can also be broiled. To broil, place the fish and all the marinade in a greased broiler pan about 7 inches away from the broiler source. Cook until the fish is tender and flaky, about 5 min on each side.