The difference is the diameter of the tubing as well as the minimum and maximum flow rates. If you are connecting this to a calcium reactor, you will want to use either LS17 or LS16 tubing as that tubing can easily be adapted to 1/4 OD John Guest style fittings for 1/4 OD tubing. The LS17 3/8 tubing has 1/4" ID and the LS16 1/4" has 1/8" ID. Both of which are relatively standard sizes which can be adapted easily for our use. As such, you should find a head that is compatible with LS16 & LS17 such as the 7518-10 head which also has the stainless rollers. An alternative head would be the 77200-50 which is similar to the ones that are coming with your pumps but support the correct size tubing.