This is a classic Bacterial Bloom. The Cloudiness in itself (the bacteria) will not harm your fish, however the reason as to why it is murky is a very deadly issue and that is Ammonia.
Since you just got the tank on Halloween and did not Fishless Cycle it for 3-6 weeks prior to adding the fish, they have started the cycle for you, but with the fish in the tank, the high levels of Ammonia and eventually Nitrites and Nitrates are most surely going to kill the fish.
What I recommend you do is perform a 50% water change with dechlorinated and conditioned tap water to dilute the toxins at the moment. It would also be wise to test the water for the 3 chemicals as mentioned every day and do the water changes daily as well to give your fish some sort of change of survival. Otherwise, it's not going to be a good out come. If you can, return the fish (since 4 Common Goldfish in a 30 gallon is far too much), cycle the tank fishless and then purchase 2 nice Fantail Variety of Goldfish.
Also, if I'm not mistaken, I think I see a small conch shell in there. Sea shells are unfortunately not good for a freshwater tank as they dissolve over time and can raise the water hardness and ph levels far too high so I recommend you take all the shells and even rocks that you didn't purchase at a pet store out.
A few tips I can give you is purchase a product called Prime by Sea chem and add this once a day (amount by directions) as it can help with the intense amounts of Ammonia. Don't mess with the filter cartridge at all for at least 2-3 months, not even cleaning (unless you absolutely have to) and in that case you just rinse it gently with old tank water. And a gravel vacuum, if you don't have one already, can work wonders.
Here are some links and good luck.
