Welcome to the site! goldfish are much more delicate than people think-but from what i see ur tank wasn't cycled!
asap, go to your pet store and look at the water chemistry. look for the api freshwater master test kit. buy it. should cost no more than $30. test the ammonia first. if it is higher than 0ppm, that means your tank is not cycled yet. since u put fish in before the cycle started, the cycle will take longer. try getting some live aquatic plants(they will help the cycle go faster, but be sure u have the right lighting for the certain plant u buy!) and remove those seashells! they release high amounts of calcium and raise the ph too high for goldfish. 2 feet by 12 feet is either a 15gallon or a 20 gallon High. too small for goldfish, but if u really like the shells u could do some research and see if u want shell-dwelling cichlids-something that would be a good candidate for that size tank and would be ok with seashells., but ive never seen them at my pet store. but let your tank cycle first!