Ur tank a bit small. Hopefully u'll upgrade to bigger tank soon. Bigger tank, less water quality problems.
Next, u don't mention a filter? Are u running a filter? Filter may be undersized, hence u have ammonia accumulation and the smell.

Activated carbon/charcoal, may help eliminate the smell. These days, (with the flowerhorn craze), fish shops often sell these red plastic balls encasing activated carbon. U could use one or 2 of that in your tank to eliminate the smell. U will have to replace the carbon every month though.