Too many fish, too little maintenance, too bad. You need to reduce the number of fish in there (sounds like it will take care of itself), buy some testkits and use them, and ensure you are cleaning the filter properly (without harming the bacteria). Make sure you are not overfeeding the tank, and you need to be doing at least 20% water changes per week given the number of fish you have in there.
Here's a tip - if you aren't willing to drink the tank water yourself, it isn't clean enough.