This is a good example of overstocking a fish tank. Don't feel too bad about it, I don't know anyone who didn't overstock their tank the first time. It's just too tempting.
Here's the problem though...
Fish waste contains ammonia, which is toxic to fish. Which wouldn't be a problem for them in a lake or river, but we keep them in a glass box. Meaning, there's no where for their waste to go.
That's why we do regular water changes. To replace some of the old, dirty water. It's also to keep the chemicals in the water balanced, or some bad chemicals can become too much for the fish to handle. I am glad to see that you're distilling your water before you use it, but I suggest going a little further and looking up (or starting a new question) how to cycle a fish tank, and why it's important.
Basically, in terms of what happened in your tank, there was a spike of ammonia from the fish waste that killed the goldfish. The ammonia came from the fish waste. Normally, in a cycled tank, the bacteria can decomposed the fish waste that isn't caught by the tank's filter. But in that small of a tank, there simply isn't going to be enough bacteria, and the tank's filter isn't going to be powerful enough to keep up.
Keeping fish requires a little bit of chemistry, and simply put, the less water you have in the tank, the less room you have for the chemicals that can harm your fish to dissipate.
Here's how I like to explain it: If you were to put a drop of food coloring in the tank you have now, it would quite noticeably alter the color of the water. Now put a drop of food coloring in a 10 gallon tank, and it won't change the color nearly as much. Put a drop of food coloring in a 55 gallon tank and you probably won't notice at all.
A good rule of thumb is for every inch of (adult) fish, you need to have 1 gallon of water. This not only balances out enough room for them to swim, but enough water to keep any dangerous chemicals from overpowering the water before you can fix it.
So, following that rule, a half-gallon tank will fit a half-inch fish. A full grown guppy will get about an inch long, and a goldfish will get about 12 inches long. That's 26 inches of fish.
You still have two guppies, which aren't going to live for very long unless you get them a larger tank. They are pretty hardy fish, so you might be able to keep them alive for a while, but it's going to take quite a bit of effort on your part. Remember, they don't have that much water to dissipate any chemicals that are harmful to them.
Here's my suggestion. Get a 10 gallon tank for the guppies, or donate them back to the fish store. They might live for a while in that small tank, but they are a schooling fish, and need at least 4 or 5 fish in the school to be happy. Not too mention you're going to be fighting too much to keep the tank clean (and the fish alive) for you to enjoy having them. And there's simply no point in keeping fish if you don't enjoy it.
So the tank is too small for fish, but that doesn't mean it's too small for anything. Most fish-keepers who get those small tanks get them to sat up a nano tank for shrimp. It's a big enough tank that you can keep a couple of Cherry Shrimp, Blueberry Shrimp, and many other interesting kinds of shrimp.
Another option, is an African Dwarf Frog. Though not very interesting in my opinion, kids seem to like them.
I knew someone who just kept a small tank of snails. Again, they don't interest me, but there are some pretty neat ones, like the Tricolor Horn Snail, or the Zebra Nerite Snail.