People here can help you, but you need to answer the questions asked. I.e. test your water and give us the numerical results.
Otherwise, we are guessing. If I guessed the problem is two fold, you have too many gold fish in two small of a tank ( even if it is already cycled) as was previously mentioned, they will overwhelm the filter system and cause the ammonia and nitrite to rise, which will end up with the death of your fish.
Secondly, is it a new tank that hasn't cycled and established a bacteria population to deal with the ammonia/nitrite/nitrate and you have 6-8 goldfish in there. The cycle is going to be very harsh and most likely result in the death of your fish.
As
@IPA mentioned at the very least reduce your fish population down to 1 or 2 then either start doing some water changes to reduce the severity of the cycle and/or add a product that has the proper bacteria to help lessen the cycle length and severity. You need a test kit though so you can monitor your water quality and know when the tank is done cycling.