Yes your tank is overstocked and if you can and the store will let you, you should see if they will let you return a few of the fish.
The general rule for a successful aquarium is one inch of fish for each gallon of aquarium size. That number of fish is going to give you nearly twice the number of fish that you should have in that size tank.
You could keep your platies and the neons or the cardinal. The Mollies will start to produce offspring if you have a male/female pair and you do not have room for that.
You also need to know about the Nitrogen Cycle if your tank is new and not cycled. It can be read at
http://www.fishlore.com/NitrogenCycle.htm. Is the tank heated and filtered?
If the tank is not through the Nitrogen Cycle you will need to be doing frequent water changes to decrease the level of ammonia in the water so your fish do not get sick and die. You will need to be changing at least half of the water every day until the cycle is complete with that number of fish in the tank as it will not take them long to become toxic with the tank being overloaded. This can take quite a long time so it would be best to try to get the fish returned if possible.
http://www.fishlore.com/Beginners.htm
There are some articles that will help give you information on how to set up and get a healthy and happy aquarium going and how to get what you need to learn about getting started in the fish hobby. We are here if you have questions and will try our best to help. Welcome to Fishlore.com.
Rose
