Yeah, get rid of that marble stuff, bury it and never let it plague this earth again. Your tank's gonna look a whole lot better and brighter w/the reef sand.

Not an expert, but I did 4 X 10 gallon water changes over a 2 week period on a 90 gallon aquarium. It may take longer for your tank. Just play it by ear.

Also test daily for ph and nitrite and maybe nitrate (if they're high). Syphon the tank and do a h2o change asap if you start to detect nitrites. That means somethings decaying in your tank and is no bueno.