You are right that the inbuilt filter on the 980 is not sufficient for marine but could be used for phoshphate removers etc, the proskim is not the most efficient skimmer in the market place but will suffice if you keep your bioload low (not too many fish) Some like coral sand and some don't as it can become a detritus trap if not kept clean, just a shallow layer for aesthetics can work well. Beach sand is fine and can compact causing areas of anaerobic bacteria which can release toxins into the tank. .