Impossible to say without pictures, but I'll go out on a limb and say it's probably not ich. Ich normally isn't just a single spot. Sounds like it was a bacterial or fungal infection, but either way (even if it was ich) it shouldn't have died within a couple hours. You might've just gotten a couple bad fish. It happens.
I won't disagree though about the QT. In my opinion, it's a "must" in this hobby to protect having to deal with introducing new diseases to an established tank.
If it was my tank, I'd just keep the water as pristine as possible, and keep an eye on everyone like a hawk. If it was ich, you should see more spot show up on the existing fish in a week or two. In the mean time, I'd get a QT up and running, and seed it with a cocktail shrimp to get it cycled. Better yet, you could put a HOB
filter on your main, or a sponge filter in your main, and after a couple weeks transfer over the seeded filter to your QT. That way you'll have the QT up and ready in case you need it.