As the others have already mentioned, you have sick/dead fish because they were going through the cycling process/ammonia & nitrite spikes. It can be inevitable that some fish will die during the process because it places a great amount of stress on them.
You say that you are not overstocked, but it is possible that you were in this case i.e. cycling and in a smaller tank. Balloon Mollies are not the best fish to cycle with, they come with their own genetic/internal problems. If fish are already in a weakened state, going through the nitrifying process can be too much for them...we never know how much stress fish have already gone through before we buy them, but I would say it is quite a lot.
As already stated, keep up with the water changes and monitor the levels every day, they will tell you when you need to do water changes. Doing them ever 5-6 days isn't enough, if you were doing them at that frequency from the beginning. You have probably already seen this sticky on cycling, but just in case...
Originally Posted by iamntbatman
-The "Fish In" Cycle
This is the least-preferred method of cycling. Essentially, it involves using live fish as your ammonia source. The benefit to this method is that you get to stock the tank immediately, but the problems associated with this method far outweigh that single benefit. Fish recommended for the "fish-in" cycle are usually hardy species but aren't always fish that you want to keep in your tank on a long-term basis so you have to deal with the hassle of removing them once the cycle is complete. Second, water changes must be performed on a regular basis (sometimes daily or even more often) in order to keep ammonia and nitrite levels low so that the fish you're using to cycle don't die. Finally, and most importantly, cycling with fish can be stressful or even deadly to the fish you're using to cycle. Unfortunately, many new aquarists are unaware of the aquarium cycle or its importance and are thus essentially forced to do a "fish-in" cycle. When using this method, you should only choose hardy species (zebra danios are a popular choice) in small numbers. Monitor ammonia and nitrite levels daily, performing water changes with a good water conditioner that neutralizes ammonia and nitrite (Seachem's Prime is a good choice) whenever ammonia or nitrite levels exceed 0.5 ppm (0.25ppm is an even safer number). After a few days the ammonia should spike. As the Nitrosomonas bacteria increase in number the ammonia level will start to peter out, replaced by nitrite. The Nitrospira bacteria will then start to grow but since these reproduce more slowly than Nitrosomonas, the nitrite portion of the cycle can take a deal longer than the ammonia portion. Eventually both ammonia and nitrite will continually test at 0 ppm and you'll start seeing a reading for nitrate. At this point the cycle is complete. It's usually best to wait a bit just to make sure there aren't any straggling ammonia or nitrite spikes but after some time you can begin adding more fish to the tank, a few fish every week or two until the tank is stocked. The most important part of the "fish-in" cycle are the ammonia and nitrite tests and the water changes that are needed whenever these readings rear their ugly heads.
Read more: http://www.tropicalfishkeeping.com/freshwater-aquarium/beginners-guide-freshwater-aquarium-cycle-38617/#ixzz1TdUrQFLt