Ok, here is what is going on inside your tank. Your fish produce ammonia, which is very toxic to the fish. Eventually, bacteria will build up in the tank which will turn the ammonia into nitrIte. NitrIte is also very toxic to the fish, and a similar thing happens, where another set of bacteria will develop to turn the nitrIte into nitrAte. The fish can tollerate the nitrate up to a certain level, which is why you do weekly or biweekly water changes; to remove the nitrAte.
Go to your LPS/LFS and buy a master test kit (about $30, but can be bought off the internet for around $20) that will test for ammonia, nitrite, and nitrate. your ammonia and nitrite should always be zero. the nitrate should be less than 40
For now, do daily water changes of about 25% until your ammonia and nitrite both are zero. Make sure you are using dechlorinator (or leaving the water sit out for 24 hours before the change), as chlorine is toxic to fish and will also kill off the beneficial bacteria.