Ah, this is new tank syndrome.
Do not worry, a lot of people make this mistake. First thing you need to know are the big three elements of a fish tank. Ammonia, Nitrite and Nitrate. When you first start a tank, there will be a lot of ammonia in the water, ammonia in high amounts is harmful to fish (harmful in low amounts to fragile fish as well). Fish waste, excess food also causes ammonia to build up.
Ammonia is broken down by bacteria that will reside in your filter, which convert the harmful ammonia into nitrite. After this, another type of bacteria will convert the nitrite into less harmful nitrates. In your case, because your tank is so new, this bacteria has not have had the chance to build up and colonize your tank yet. As a result, your fish have died from ammonia poisoning because there is nothing to break the ammonia down.
You can read more about new tank syndrome here http://www.aquahobby.com/articles/e_syndrome.php
Meanwhile, I would suggest you try to find another home for the 2 discuss and goldfish if possible. What you need to do is a fishless cycle, which means just leave your tank and filter running for at least 2 weeks with no fish. (1 week is possible for very hardy fish, but I would suggest 2 weeks minimum just in case)
An alternative is to buy more hardy fish that can withstand the cycling period, like the zebra danio. Do not worry if the water becomes cloudy during the first 2-3 days, it's natural. When I started my first tank the water was cloudy for almost a week before it cleared up.
As for the smell, I'm guessing it is because of the dead fish. I have no experience with discuss or goldfish but they sound like a bad combination, I would not suggest to put them together. The LFS guy you talked to may just be trying to make a sale.
My suggestion would be to take the fishes out of the tank, do a full water change and start a fishless cycle.
If you really can't get the fishes out, I would do a partial water change and hope for the best. Remember to dechlorinate the water before putting it into the tank. Also, there are plenty of resources on the net regarding starting a new tank, remember to research on fish before deciding on them! Sometimes you just can't simply trust what LFS people say, it pays to be careful. Hopefully the other AQ members can give you better advice, as I'm not as experienced as them.
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