new tanks will give off a smell due to the cycling process. If the tank has been established for a while, then its most likely the ammonia leaching an odor. Tap water also contains other materials and chemicals we dont normaly test for, which can aid in the odor. i would think doing a water change and using some activated carbon (which absorbs odors and colors) will clear up the smell in no time. Honestly though, try getting a better source of water, tap water, no matter the location, should be your last resort in saltwater setups. good luck
