what do you do when your pet fish dies?

The problem isn't your "janitor fish", they're usually very peaceful. The problem is you have too many fish in too small of a tank. A 5 gallon may seem large, but in all honesty, it's really not. The only fish I would keep in a 5 gallon tank, would be a single betta. And, that's assuming there's a heater in the tank as well to keep the water temperature stable and warm. When you overstock a tank with too many fish, the water quality becomes poor. Ammonia builds up which is very toxic to fish. It damages their gills, and usually they'll end up dying. Do a search online for the nitrogen cycle or cycling a fish tank. It explains the whole process.

I have to agree with the others you had too many fish for such a small aquarium. I would be willing to bet that your nitrates and ammonia levels were off the charts. I would recommend for a five gallon aquarium guppies or small neons or cardinals. plus your janitor fish.