Hi warhawk and welcome to our freshwater beginners section!
I see lots of members have helped you with your situation so far and are doing a good job! As you've seen, ammonia and nitrite(NO2) poisoning are a big deal in fishkeeping and it takes a while to figure out that large water changes are your friend in most cases! Good test kits are also an essential friend of beginners.
One other thing may be going on in your case. No one explained the importance of tap water versus these other sorts of water, distilled and bottled. It is nearly always better to rely on tap water as the source for your aquarium. There are extreme cases where tap water presents too much of a problem for the goals of the aquarist, but those are rare.
Tap water contains trace minerals (Calcium, Magnesium, tailing off to smaller and smaller amounts of many other substaces) that serve both individual positive purposes and also collectively create a "hardness level" that ideally serves as a steady baseline for your fish. When water evaporates, the water molecules leave the tank without taking other substances with them, thus they leave behind these minerals (and hundreds of other types of organic and inorganic molecules.) If water is simply topped up, these minerals will make the water too "hard" for your fish to be able to regulate with their osmotic systems. But if appropriate water changes occur, your water will be maintained at mineral levels similar to your tap water and your tap water will remain a source of water that will not "shock" your fish.
Distilled water by definition has very little or none of these minerals. Bottled water is usually water with a mineral content very different from your tap water, but basically unpredictable and potentially changing, depending on source. Neither of these is usually acceptable as a source water for aquariums.
To top it all off, some of the fish your LFS has allowed you to try as a beginner present individual problems with respect to water chemistry. Let's look at some examples. Mollies often need very, very hard water as their ideal environment, in fact this is often mistaken as a need for salt! Neon tetras really prefer the extremely soft, acid water they come from in the Amazon region of South America and additionally are fish that really do better in tanks that are at least 6 months old, rather than a new tank. Guppies, despite being rather easy fish to keep overall, are not particularly hardy as first fish (neither are neons or mollies for that matter.) Not to let my comments get too far out of perspective, all these are popular commercial tropicals and will usually do ok in a variety of water conditions if the conditions are stable, but point is just to give a glimpse of some of the greater detail one begins to get into as a hobbyist, which is the point of these forums.
All these things, learning about ammonia and nitrite as poisons to be measured and watched, learning about the functions of filters and how they need to be "cycled" prior to being ready to support fish, learning about the importance of water changes and of the parameters of the water itself, learning about how to create healthy "stocking plans" for your first fish are all the stuff of the basic beginner freshwater world we live with here in our subforum. The members here are all learning about it at different levels, even us old guys to the hobby and there is a great exchange of info. Keep at it and things will quickly start to get better and make more sense.
~~waterdrop~~
