Why are my fish dying?

Question
Dear Brittnay,
I got an aqaurium for Christmas from my parents, So I set it up with a whole bunch of different fresh water Tropical fish, Including a rainbow shark which gets along with everything, but in the past couple of weeks a lot of my fish are dying with no explicable reason. I even looked up information of common fish diseases and got common antibiotics for fish and an isolation tank if I need to treat the fish. All the fish that died showed NO symptoms of disease or injury and I never have stressed them. Do you have any insight for what could be happening?

Answer
Hello Emma,
There are some vital pieces of information that are missing that I have to know in order to be able to help you better.
First of all, I need to know the size of your tank. Water volume and surface area greatly dictate how many fish you can have, and of what kind.
Also, I need to know the temperature your tank is kept at, how often you are doing water changes, and your feeding routine.
It would help a lot also to know what kind of fish you have in there.
You always need to know what kind of fish you have in your tank, so you can be sure you are providing the proper home for them.
Remember, fish come from all over the world so they all grow very different lengths, have different diets, and are compatible with different kinds of fish.
While you are gathering this information, please also look up the Nitrogen Cycle on the internet, this is very important for a fish keeper to know.
Please write back with more details and I will be more than happy to help you.
As a side note, Rainbow Sharks grow a little over 6 inches long, and are  very aggressive with others of their own kind. With this kind of fish in the tank, plus "a whole bunch of others" as you say, you should have a 30+ gallon at least.