Well, if yours is what I think it is. It's really a catfish. There are few true sharks that are kept in the home aquaria. Dont sweat that part...as I said earlier. I have rainbow sharks. They are not true "sharks" either. The problem is, catfish get huge!! That fish will probably grow up to 4ft in length.
Hate to say it...but its best if you return the sharks. A tank that small will quickly stunt their growth, which can be very harmful to the fish.
Now on to the "golds" as you say. I'm pretty sure were all guessing these are goldfish. If it is in fact a 15 US Gallon tank.
See Here You only have room for 1 goldfish, and thats pushing it. 1 Goldfish should be kept in a minimum of a 20 gallon tank.
So, lets try to take care of these problems one by one. We all want to see you have a successful tank so you can watch your fish grow and prosper.
Again, any more questions you have, feel free to ask. Were all here to help you out. Even if you question something myself or someone else has said...bring it up...its best if you are 100% clear on the answers your getting.
Thanks for joining, and good luck!
-Jay