I am guessing you have the Diablo 350 (XS350) skimmer... http://www.coralvue.com/diablo-350-protein-skimmer
If that is the case, then you need ONE HY5000S pump with the adapter kit... You can purchase it directly from here...
http://www.coralvue.com/reef-octopus-bubble-blaster-5000-skimmer-pump