102-301 Softenizer 20 for 10mm
102-302 Softenizer 20 for 13mm
102-311 Softenizer 50 for 10mm
102-312 Softenizer 50 for 13mm
102-313 Softenizer 50 for 17mm
102-322 Softenizer 300 for 13mm
102-323 Softenizer 300 for 17mm

As Softenizer intakes water from outflow tube of an external canister filter, you need to connect right size of adapter depending upon the diameter size of outflow tube. Be sure to check your tube size (10mm, 13mm, or 17mm diameter are available.), when purchasing Softenizer.

Softenizer 20 for less than 60cm aquarium tank, or for low water hardness level
Softenizer 50 for 60-90cm aquarium tank, or for medium water hardness level tank
Softenizer 300 for over 90cm aquarium tank, or for high water hardness level tank