You say your clown is lethargic, what is it doing?
Is it still eating?
As long as that maroon is in the tank, I would not add any other fish. Consider the tank stocked.
You either has some ammonia issue, disease (timeline of deaths would indicate velvet), or the clownfish being aggressive and killing other fish.
You didn't witness any aggression? What did the dead fish look like? Tattered fins?
You can either take the clown fish back to the store, let the tank sit with no fish for 76 days, they start trying to restock. I'd recommend quarantine before adding to the tank.
Or you can watch the clown and see if he is going to need to be removed for disease treatment. Watch for behavioral signs, hiding, swimming into powerhead, not eating, yawning, head twitch, flashing on rocks sand.
That clown should be you only fish though imo, she'll claim that whole tank as hers