Cut six large pieces of aluminum foil and lay them flat on a clean workspace.
Place one tilapia fillet directly in the center of each piece of foil.
Season each fillet evenly with the lemon-pepper seasoning, drizzle with the fresh lemon juice, and top each piece of fish with exactly 1 tbsp (0. 5 oz / 14g) of butter.
Gently fold and crimp the edges of the aluminum foil over the fish to form tightly sealed packets.
Place the six foil packets into a 6-quart slow cooker. Secure the lid and cook on the High setting for 3 hours, or until the fish reaches an internal temperature of 145°F (63°C).
Carefully remove the hot foil packets from the slow cooker using tongs. Gently open the crimped edges to release the steam and check that the tilapia is tender and flakes easily with a fork.
Serve immediately while hot.
Add Aromatic Extras: Elevate the flavor profile by laying down a bed of thinly sliced fresh lemons or adding a sprig of fresh dill or parsley inside the foil packet before sealing. The aromas will infuse directly into the fish as it cooks.