Vinegar will do the trick also.. Stick it in a spray bottle and spray the tank. If you have a heavy buildup, use some baking soda also (will require a little elbow grease).. Above is purely organic and residue will hurt nobody! Have a dealer around here that makes a product called Rhino. This stuff is so strong you can take oil stains out of your driveway.. And if you happen to ingest it, probably get heart burn at the worst. Totally organic and safe stuff also.