I acquired an Eheim Pro II from a friend and he had been running saltwater for 3 mos with this filter. The canister has been dry for 2 mos. since. I want to use this for a freshwater tank now, can I just flush the medium for a few days and reuse the medium, Ehfisubstrat. I will buy a new filter pad and carbon though. I just hate to waste the medium if I can reuse it.