:  Salt-Water Quality
Temperature 72°F to 80°F
Salinity 1.021 to 1.025 g/ml fish only, 1.023 to 1.026 g/ml corals
Nitrate Fish: < 50 mg/L, Corals: 0 to 10 mg/L
KH (Total Alkalinity) 8 to 14 dKH, Ideal = 10 dKH
Phosphate0 to 0.03 mg/L
Ammonia0 mg/L
Nitrite0 mg/L
Calcium380-500 mg/L
Magnesium1200-1400 mg/L
pH8 to 8.3
Water Replacement: Because of the gradual and significant evaporative water loss from an aquariums system, water must be added to maintain salinity at 1.024 g/ml. The water that is added should be filtered and not poured directly from any tap source. Municipal water treatment generally involves the addition of chlorine (Cl2(g)) and chloramine (NH2Cl(aq)) to destroy viruses and kill microbes. Tap water chlorine levels are usually at least 3 mg/L and chloramine levels are at least 1.0 mg/L. Chlorine levels greater than 0.002 mg/L are deleterious to marine life. In addition, tap water contains dissolved solids, dissolved minerals (including those from house plumbing tubing), phosphates, and other impurities. It is important that all of these be removed before adding tap water to a marine system. Also - tap water from the hot water plumbing should NEVER be purified for addition to a marine aquarium system. It has more dissolved metals than the cold water system, which will shorten the life of the purification system.

Reverse Osmosis: This purification system uses high water pressure to push water through a barrier that other solutes cannot pass. The cellulose acetate or polyamide polymer membranes of these systems have very small pore sizes (1.5 nanometers) that will reject dissolved substances but will allow water to pass. These systems produce water with substance removal rates of 90% to 99.5%. The only drawback of this system is the waste of water - the water that does not pass through the membrane containing concentrated solutes is discarded. Most reverse osmosis systems use pre-filters to remove dissolved solids and prolong the life of the purification membrane. They tend to be cheaper to operate and require less maintenance than other purification systems.

Deionization: This purification system uses resin beads impregnated with very small pores (2 to 5 nm). To make the beads deionize, they are exposed to either strong acids or strong bases to load them with hydrogen or hydroxide ions. The beads are loaded into a canister through which tap water is allowed to flow. As cations (positive ions) and anions (negative ions) of tap water flow between the beads, they exchange ions with them. The principal ions in tap water - calcium, magnesium, sodium, potassium, copper, lead, nitrate, bicarbonate, sulfate, chloride, and iodide - end up stuck on the bead surface. As the beads becomes saturated with tap water impurities, their pH and color change, indicating they should be recharged or replaced. Tap water passed through a deionization system can be up to 99.5% free of dissolved ions. Pre-filters may be needed to prolong the life of the resin beads. Carbon reactors may be needed to remove organic impurities.

Final Notes:
  1. Tap water should not be used to replace evaporated salt water in an aquarium.
  2. The most important aspect of aquarium salinity is stability. As water evaporates from an aquarium, salinity rises. Removal of saltwater, salt spray, and protein skimmers lower salinity. Barring leakage, evaporation exceeds salt spray in most systems and salinity gradually falls over time. Any and all adjustments must be made GRADUALLY. Saltwater fish can tolerate salinity that it not optimal, but what is often fatal is rapid salinity change. The addition of water or salt to an aquarium system should be done gradually to avoid wide fluctuations in salinity.