One possibility is that the chloramine in the water killed off your good bacteria and maybe some of the first fish that died. Then once you got rid of the chloramine, you didn't have enough bacteria left to process the ammonia the fish were producing, so then ammonia levels shot up and killed some more fish.
If all you keep in there is the single cory, 3 water changes a week should be sufficient. You don't really have to hold back feedings that much, I'd at least feed him before each water change. If you feed lightly, you shouldn't have any issues with feeding him daily though.
Do remember, when things level out and you start to add fish again, do it slowly. One or two at a time. That way you won't have more huge ammonia and nitrite spikes. A test kit is a good idea too. It's really best to be able to monitor ammonia and nitrites so you can do more water changes if necessary.
If all you keep in there is the single cory, 3 water changes a week should be sufficient. You don't really have to hold back feedings that much, I'd at least feed him before each water change. If you feed lightly, you shouldn't have any issues with feeding him daily though.
Do remember, when things level out and you start to add fish again, do it slowly. One or two at a time. That way you won't have more huge ammonia and nitrite spikes. A test kit is a good idea too. It's really best to be able to monitor ammonia and nitrites so you can do more water changes if necessary.
