Tank and nitrate levels

Question
QUESTION: Hello. I have set up a 55 gallon tank with eight neons in there for two months.
The neons are fine but I've lost about fifteen other fish of all types  within 24hours
Of being in the tank. I test the water and do water changes each week but my fish keep
Dying. Any idea what could be the problem?   Thank you.

ANSWER: Kevin,
Are you buying your fish from the same store? If so, try a different one since they do not all use the same breeders. What are the levels of ammonia, nitrite, and nitrates in the water? Is your Ph off a little? Are you using a water conditioner every time you do a water change?

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

QUESTION: Hello  no I'm not buying fish from same place and PH is good ammonia is good also nitrites are good
Only the nitrate is slightly different . Could it be the gold colored gravel or not enough bacteria in the tank.  Help. Thanks.

Answer
When you say the ammonia and nitrites are good, what does that mean? What are the levels? If you have had the tank up for 2 months, the tank has cycled and has all the stuff it needs to keep the fish healthy. The color of the gravel should not be the problem. Did you cycle the tank, or did you set it up and put fish in right away? Have these fish all died at the same time, or just a few here and there? There are so many things that could contribute to the problem. What are the other fish that have died? Are the tetra's all looking alright with no noticeable illnesses? When you clean out the tank, it is best to use a gravel vacuum. This will remove the uneaten food and waste quickly.