The easiest way to clean your bubbler is to boil it. You can also try cleaning it with soap and hot water. Don’t forget to rinse it with hot water to remove the soap from your bubbler pipe.
In order to get the best results, you should use isopropyl alcohol that is also known as rubbing alcohol. You can get it from drug stores, convenience stores and even from some grocery stores. Take a plastic bag (I suggest to use Ziploc bags) and fill it with isopropyl alcohol, warm water and add some salt. Put your bubbler pipe in the solution, close the bag and shake it so that the solution goes into your bubbler. The solution will become dark because of the dirt that was in the bubbler. You can empty the bag and make more of this solution and repeat the whole process until the solution stays clear. After cleaning your bubbler with this solution, rinse it with hot water. I suggest also boiling it in hot water. Now you have a nice clean bubbler pipe that looks like a new.