fish dying

Question
QUESTION: I recently got some fish and they have been slowly but consistently dying off.  I did  cycle the water before adding the current fish in and have tested it and have the correct levels of bacteria and nitrates. Unfortunately, the fish have started to die.  At first only one did. The next day two did, and now in the past week I have lost 8 fish.  I don't understand because the water is cycled. Do you know what it could be and what I can do to prevent future deaths?
Thanks

ANSWER: Hi Tim,
Explain to me how you cycled your tank.  There is no way to measure the amounts of bacteria.  And there is no correct level of nitrates.  In order for a tank to be cycled, the readings must be 0ppm Ammonia, 0ppm Nitrites, and with some readings of Nitrates.  I believe it is the water quality that is affecting the fish, otherwise you would not be losing so much fish in such a little amount of time.  You test for Ammonia, Nitrites, and Nitrates with liquid testing.  Do not test with test strips as they are very inaccurate.  Explain to me how your tank is set up, and what you did to cycle it and I will respond with an explanation and advices.
-Matt-

---------- FOLLOW-UP ----------

QUESTION: Hello Matt,
Thanks for taking time to respond. It is a 30 gallon tank with an under gravel filter and a AquaClear Power Filter.  I used 5 goldfish to cycle the water for 3-4 weeks.  Then after that i brought a water sample to the local fish store and they tested it and said that the water was not harmful to the fish so I started buying fish for the tank.  I bought about 6-9 fish per week. A couple of them died immediately, before they got into the tank, but I thought nothing of that, I figured they were just sick or something.  After a couple of weeks a couple started dying.  After about 5 or 6 died i brought some more water to the fish store to be tested and they said the water was still good, they did test the water with liquid testing, not strips.  So here I am, confused.  Thank you for your time and advice.
Tim

Answer
Hi agian Tim,
If you are cycling with fish, 3-4 weeks is not enough time for a tank to cycle.  Fishless cycling is the fastest way to cycle and is a lot faster compared to cycling with fish and even then, it takes fishless about 4-6 weeks to cycle.  I believe the reason why your fish is dieing is because of Ammonia intoxication.  Their liquid testing... must be out of date or not working.  With some Ammonia liquid testing, such as API's, the testing shows 0ppm when the drops are initially added, and you must wait 5 minutes for color to show up.   With the goldfish giving off ammonia, including adding 6-9 fish per week, sounds like ammonia overload to me.  Also because the tank is considered a smaller tank, intoxication would happen more easily compared to a larger tank.  In order to keep the fish alive, during a cycle with fish, you must do a 20% water change every other day to prevent the ammonia and nitrite intoxication.  After you have grown the 2 important sets of beneficial bacteria, the ammonia-to-nitrite and the nitrite-to-nitrate converting bacteria.  Then you can resume water changes to only once a week.  Nitrate is far less poisonous than ammonia and nitrite.  But in order to grow these 2 bacterias that break ammonia and nitrite down, you must allow time for it to cycle.  It is called the Nitrogen Cycle.  If you dont understand it or are confused, research it on google, or ask me with another followup question and I will go into further explantion.  
-Matt-