Same pump with supposedly different impeller. With that said however. I have experienced and others have also the 4s skimmer pumps coming withe the water impeller. I'm sitting here with a water impeller in a box after having to purchase the correct impeller for my skimmer pump. Personally I recommend alternative to the varios pumps seems like every new pump is neutered to upsale you (like how coralvue is recommending a 4s skimmer pump to replace the older 3500s pumps. If your after a return pump go with a jebao dcp I'm running a dcp 10000 on my 150 gallon reef tank drawing 41-42 watts on setting F065 and setting up a 90 with a dcp10000 m (wifi model). The old diablo skimmer pumps and the old reef octopus dc3500 pumps were made by jebao and I for one belive so are the varios pumps