I have a pretty well filtered new reef tank, at 42 gallons with a 10 gallon sump includes protein, scammer, and reef mat. I use the word scammer pejoratively, because I measured the nitrate and nitrite levels in the skimmer waste cup and compared it to that of the reef tank. My reef tank nitrate and nitrite values are quite low, but the amount of nitrates and nitrates in the skimmer cup are rather high. The long and short of that is, against my initial uneducated belief that the skimmer is a money grab scammer was born out of ignorance . The skimmer is not scamming me, but is scamming nitrate and nitrites. . besides that, Red sea advised me to put some charcoal pellets in me sump. I ordered some and they came in these black threaded bags I think each bag is about 2 pounds, (I imagine many users here are familiar with these) and I threw my first bag in the back of the sump. quickly it caused my reef mat rolls to go completely pure o found black powdered detritus on some of my aquarium decor. that just didn't seem right and concerned me that that a amount of charcoal on the matt rolls could prevent the matt from absorbing what it is supposed to. so I backed off and took about 75% of the pellets out of the bag for total charcoal bag of about 6 to 8 ounces. Seems like a small amount for 52 gallon system, but also the system is quite well filtered as it is. I suppose I could research it more, what is activated charcoal attracting and collecting that my matt and protein skimmer miss. . But also is there a rule a thumb on how often one should change activated charcoal bags , and really what would be the appropriate amount for a 51 gallon system? any thoughts? I appreciate everybody's thoughts on these questions that a newbie reef owner ponders in the whole New World of reefing that I have discovered this last six weeks that intrigued me beyond what I ever thought.