Dosing and Fluid Measuring System (DOS) One of the most intelligent dosing pumps available is the Neptune DOS Fluid Metering System. You can program both pumps separately for any type of dosing your tank may require because it will connect straight to your current Neptune Apex, ApexEL, or Apex Jr. Although it is not restricted to two-part dosing, the DOS pump has two pump heads for the most popular type of dosing. You can use it for liquid food dosing, minor trace element additive dosing, automatic water changes once a week or even once a day, and pretty much any other liquid that needs to be dosed in metered amounts in your tank. All you have to do is hook up the DOS Fluid.