worried about sick fish

Question
I am a frustrated novice and am finding having a tank a lot of trouble.  Here are a few of the things I never knew; that you should not add the fish water from the pet store to the tank.  I was just cutting a hole in the plastic bag and letting them go.  Also, I had a black molly for a week, it got sick and died and had 2 babies, and although I bought the separate baby breeder box, they died as well.  I feel some days that this is over my head.
We've never had sick fish that showed outward signs of illness, but many that have died within days of being brought home from fish store.  

Answer
Hi Martha,
Sorry you have had such a hard time keeping fish.
First of all did you ever allow your aquarium sufficient time to cycle? If not, then that probably caused the majority of your fish problems. They will not show any outward sickness but it is the high ammonia that kills them. I don't know if you are familiar with the process of tank cycling. Sometimes the pet stores don't explain it very well and it isn't your fault. You -must- stock your tank very slowly with no more than 2-3 small fish per week otherwise, you will have too much ammonia and nitrites for fish to survive.  The best way is to start out is filling your tank with dechlorinated water, (and be sure to add enough since some tap water supplies add more chlorine and chloramine than others) and it also helps to add Cycle or some other brand of beneficial bacteria as extra help for the ammonia problems. Then you can add one or two very small fish just to get the cycle started, the small fish will produce the ammonia the bacteria need to start colonizing your filter and gravel bed. When I say fish I don't mean goldfish or any other fish that is considered dirty fish. Get a Black skirt tetra(1) or maybe  Zebra Danios(about 3) of course if you have a really large tank you might want to get just a few more. This amount would work in a 10-20 gallon tank. Check your water with a water test kit regulary to be sure that ammonia and nitrites aren't getting too high. You will have to do partial water changes during this time. Make sure that the replacement  water is the same temperature as the tank water. When your tank water tests show the ammonia and nitrite levels to be at 0 then you can add the fish you wanted in your tank. Only add them a few at a time with at least a week between additions. That gives your bacteria in your tank time to adjust to the extra bio load from adding more fish.  You will need to establish a regular weekly water changing schedule. Change at least 1/3 of your tank water at least once a week and use a really good water conditioner and dechlorinator. Be sure to vacuum the gravel too with each water change to remove any waste or decaying food. Don't overfeed your fish since that will lead to very bad water quality and cause your fish to get sick. Be sure when you do start to stock your tank that you have researched the grown size of the fish and temperament of the fish since so many don't get along with each other and many out grow the size of the tank they are in or it just gets too crowded. Also, many fish are very territorial and you have to have a very large tank for them to establish their territory or they will fight and even kill each other.
If you are really careful to give your tank time to cycle your will be successful with your fish keeping. Don't give up, just be patient and it will work out.
Feel free to ask any more questions as you set up your tank.
Hope this has helped,
Karen