Possibly of more concern is the export of calcium by all of the exports. Significant amounts of calcium are is found in all of the exports; skimmate contains about 2200 ppm calcium, sludge about 37000 ppm, Caulerpa about 1743 ppm and Xenia about 3350 ppm. Put another way, the analyzed sample of skimmer sludge was 3.7 percent, by weight calcium. Harvesting a pound of skimmer sludge removes about 17 grams, over half an ounce, of calcium the system. If all of this calcium was present as calcium carbonate, and it is doubtful that it would be, then removal of a pound of skimmer sludge would remove about 42 grams, or about an ounce and a third of calcium carbonate. The average tank in the Tank Water Study had a volume of 191. 3 liters and contained a calcium concentration of about 400 mg/kg. Assuming a specific gravity of 1.025, this means that the average tank contained ((191.3) x (1.025)) or 196.0825 kg of artificial sea water. This water, in turn, contained about (196.08 x 0.4) = 78.4.grams of calcium. In other words, removal of one pound of skimmer sludge, would remove 17/78.4 or about 21.7 percent of the calcium in the sea water of the tank. Skimming enough to build up skimmer sludge can be a major way to remove calcium a tank. In tanks with continual supplementation of calcium, such losses are like not to be noticeable, but in tanks with periodic supplementation, provided they also are accumulating skimmer sludge, the sludge may noticeably be removing calcium. The bacteria in the sludge, or inorganic precipitates, perhaps of calcium phosphate, trapped in the sludge may be responsible for much of the drop seen in calcium concentrations.