what can I do about a smelly fish tank?

Adding fresh activated carbon to the filter will help remove odors from aquariums. Making sure you have adequate filtration (10x the output of the tank per hour) will also help. Doing thorough gravel vacuuming and removing any dead plant leaves or other decaying matter can also help. If the tank is very dirty, doing 10% water changes and thorough gravel vacuuming for a few days can help to get the water quality back up to good levels, after which you can drop down to one 40% water change per week.

Incense

Clean it of just keep smelling the dirty tank. If this dirty how do the fish survive?

Clean it!