How do I stop my fish tank from smelling?

I have a 35 litre tropical aquarium, running at 26 degrees Celsius, and I've had it for about 3 days. I have two filters running in it - an Elite Stingray and a Fluval U2. It has about 10-15 fish in it and I've noticed that it is beginning to make my room smell a bit. Does anybody know why this is, and how I can eradicate this?