Hi, and welcome...
The first thing I can say is that you should probably expect to lose more fish. That is too many fish in that size of a tank, and way too soon to have a fully stocked tank. Did you cycle the tank before putting fish into it?
In order for someone to help, we'll need to know the water quality stats for ammonia, nitrite, nitrate, and pH.
The first thing I can instruct you to do is to purchase some polyfilter if you have it available to you. If not at your LFS, you can find it at http://drsfostersmith.com
This will work much faster than carbon to suck the medication out of the tank, and to help soak up waste levels at the same time. The polyfilter should be changed after about 48 hrs, and a fresh piece put into your filter for another 48 hrs. Once this is done, the water will need to be tested again. Most important is to get that medication out of the tank asap without shocking the fish with too many or too large of water changes. Small, daily changes of 10 - 15% for a while will help a lot.
If your tank is cycling, which is highly likely at 3 wks old, medications can be deadly. Coppersafe is not a safe medication to use for all species of fish. Angelfish are sensitive to copper, as are most catfish and shark species. Until we know it's safe, I'm going to say it is best NOT to medicate this tank.
In addressing the stocking of this tank, angelfish grow to 8 inches in diameter for average, and are aggressive and territorial. You mentioned catfish, can you tell us what kind? It would help a lot if you could post pictures of your fish here for us to see, so we know what species we are dealing with. Angelfish also tend to eat neons, especially in a crowded tank. Shark... what type? These also are known to be prone to stress and illness during cycling, in crowded situations, or if not enough territory. The smallest of the freshwater shark species readily available is the red tail shark, and you are again looking at a fish that reaches 6 - 8 inches long... then there are bala sharks which average 14 inches... and irridescent sharks which average about 3 - 5 feet.
How often are you doing water exchanges? How much water at a time? How often are you feeding the fish? How much each time? A temp of 75 is borderline for most of the fish you have, and would be better at 78 degrees.
The more info about your tank and fish that you can provide, the faster we can all help you here. Pics are important if you can get them!
The chances are extremely high right now that your fish are suffering from poor water quality, not enough heat, overcrowding ( which causes stress) and from being mismedicated. How much territory (decorations and plants) is in the tank?
As I said, the more you can tell us, the faster we can help.