The hayward EC 65 is a DE filter and is rated at 25,920 gallon of water in a 8 hour time. Which means in a 8 hours it can filter 25.920 gallons of water. If you run it for 10 hours it is 32,400 gallons. This filter will work with your pool.
It sounds to me he wanted you to buy a cartridge filter but said DE instead. DE filters are the best filters availble and the EC65 is Haywards time tested filter
Thanks for the quick reply. He did want me to buy a DE, but he's saying the one I received is a cartridge and that cartridge filters cannot be D.E. filters. I think he's just not familiar with this model. I don't know what to believe. I really don't want to return this if I don't have to-- I'll lose about $100 in shipping.