There are several good points here, with such a small tank any small contaminator could cause problems, or if dechlorinator was forgotten or enough wasn't used that could be a problem. I've seen mixed opinions on the salt thing, some users (as you can see) advocate NEVER using salt unless you have specifically salt water fish, others say a little salt is OK. I've been told by a number of people that guppies don't mind "brackish" tanks. When I was having sick guppy problems, I read about doing "salt dips." At the time I didn't have aquarium salt, so I used sea salt (which is not iodized, I think that's important, because table salt IS iodized). I did this until I got to the fish store to get aquarium salt. Interestingly enough the brand I bought was by API and is labeled "freshwater salt." I now add that salt to my tank water on a consistent basis. According to the label for "fresh water" fish, I add about 1 tsp of the salt per gallon. I've been doing this since January of this year and haven't had any problems with my fish. I'm not sure how much my plants like it though. :-/
Another thing I might recommend is preparing your water in advance for the water changes (if you don't already do so). The dechlorinator needs a little time to work, plus if you let the water sit out it will naturally allow the chlorine to evaporate on it's own. There are also many opinion on this process (sometimes called aging your water). Some people recommend continuous aeration of the water while it's sitting out, others just let it sit. My aunt used to use "aquarium grade" buckets to allow her water to sit out. I have a small home and not a lot of room for buckets, so I personally use a slightly different process. I've read about other people doing this, and it's worked for me for a couple months now. When I finish a gallon of milk, I thoroughly and immediately rinse the container with hot water, and add a bit of bleach to the inside of the container. I put the lid back on, swish the bleach around inside to cover all surfaces. Then I rinse THOROUGHLY and allow to air dry. I usually smell the inside of my now "clean" container to make sure it doesn't smell like bleach after it's dried. Then I will put a bit of dechlorinator (and my salt) in the gallon and fill it up. I let it sit out with no top for a couple hours, then put a top on it and store it in the closet or under the table until I need it for water changes. I only clean the container the one time, I just keep refilling it as mentioned above each time I use it. Anyway, I've probably given you way too much info, and I'm not even sure if it's considered to be "correct" info, I just know that this works for me.