why are my Goldfish staying at the bottom of the tank?

Test your water with a proper liquid test kit, the most toxic properties in water are invisible! It can look clean but still be poisonous.

As the tank is new and you have very messy Goldfish in it I am presuming you haven't cloned the filter and the tank is now going through the nitrogen cycle, the main part of which includes a massive ammonia spike which burns a fish inside and out causing lethargy, gasping and ultimately death.

Test your water, do a 50% water change, check your tank is big enough for Goldfish (they need massive over-filtered tanks) and have a read of this:

http://www.fishforums.net/index.php?showtopic=1753...

Usually they stay at the bottom of the tank if the temperature of water is too cold for them.

I think about the setup the fish are either warmer at the bottome of the tank or they feel more safe down there.

Just because the water looks okay it may not be buy a test kit and test for ammonia nitrites and nitrates

Without knowing anything else about your setup, I would assume that they simply feel safe there and are taking their time adjusting to their new ecosystem.