IMO, your tank most likely hasn't cycled and this may be contributing to the smell. I would highly suggest getting a test kit that will give you levels on ammonia, nitrites, and nitrates. Basically, the smell is most likely coming from the built up ammonia. Cycling involves beneficial bacteria that builds up and helps convert toxic amm & nitrites to less toxic nitrates. The 1st type of bacteria converts amm to nitrites. The 2nd type converts Nitrites to nitrates. Since you have fish in your tank I would suggest watching your levels very closely (test daily) and if your levels of nitrites/amm gets to the higher end of the test, do a water change.
First you will see a huge spike in amm before the 1st bacteria kicks in and then the amm will drop to 0ppm and nitrite will start to get higher. When the nitrite has spiked, the 2nd type of bacteria will kick in and your nitrites will drop to 0ppm and your nitrates will probably be pretty high. At this point your tank is cycled. You can now do a 50% water change to lower the nitrates (which at high enough values can be toxic) and start adding fish. But be sure to add slowly in order to maintain the cycle!