Wash potatoes in skins.
Drain but do not dry.
Place in slow-cooker, cover and cook on LOW setting for 6 hours.
Lay fillets, cut into serving size pieces, on top of potatoes.
Dot with butter and sprinkle on dill.
Cook on LOW setting for 1 hour, or on HIGH for 30 minutes.
Fish should flake easily when fork tested at end of cooking time.