What sized tank, what other tank mates? Anything new added recently? What are the ammonia/nitrite/nitrate readings?
47 fish dont generally die for no reason - so with more info someone might be able to help and prevent any more dying.
Sounds more like poisoning than illness if it were sudden, maybe poor water quality or a change in the water coming into your house, or has anything been sprayed in the area or dropped in the tank?
You say their tank was medicated - what medecine have you been using, and what dose?