This video talks about the basic options you have when you realize your tank is overstocked.
- Buy a new tank : ) (look for deals on craigslist or your local classifieds, look for the dollar per gallon sale at Petco.
- You can use a large plastic storage bin with filter and airstone for temporary housing until you can upgrade your tank.
- You can try to re-home some of you fish or return them to the pet store.
The above options are the best. If for some reason you absolutely cannot upgrade or are unwilling to re-home your fish, the best possible thing you can do is frequent and large (80%) water changes. This is only a temporary solution. You will absolutely need to find them appropriate housing or re-home them eventually. But water changes will help to keep them alive until you can figure out a way to properly care for them.