From Marine To Freshwater

[nanonewbie]

hi all, have just purchased a 2nd hand marine tank (boyu TL450), it was used for a period of quarantine before previous owner added new fish to big set up, so nothing major...basically it comes as a ready to go tank-but i need to get the filtration sorted out-it has an internal filter system which i need to fill with media/sponges.
Any advise on the changing of marine to freshwater will be much appreciated :0)
thanks in advance, nikki