Generally speaking, our goal is to manage in a relatively small closed system, a stable environment that mimics the natural environment (a very large, open system) of the animals and plants we are keeping — think, Great Barrier Reef. Note though that wide variations appear to all “work” successfully.
Where to begin
Start fresh: In the absence of safe and reliable natural saltwater (NSW), we have to make our own or buy it. Because most tap water has all kinds of “stuff” harmful to a closed saltwater reef system, start with pure fresh water and build from there.
The most popular approach is to use an RO or RO/DI system to make pure
freshwater in a food grade plastic container. Then, add-in a commercial mix to get the desired saltwater concentration (aka specific gravity) with the other required chemical components (calcium, magnesium, etc.). About 1/2 cup of salt mix yields a gallon of saltwater.
We use this artificial saltwater (ASW) to start the system and for periodic water changes.
Use RO/DI water to replace water lost to evaporation — often called “top off”. Freshwater evaporates leaving the salt behind so the salinity level will increase unless freshwater is added. Depending on the size of your tank and other factors, figure about 1 gallon/day.
Chemistry
Debates here also.
Salinity
FOWLR: 1.020
Reef: 1.025
pH
subject to change between collection and testing; can be done accurately and inexpensively at home
FOWLR: 7.6 to 8.4
REEF: 8.0 to 8.4
Ammonia (NH3-4)
Natural Seawater Value: 0.010 mg/L
Acceptable Range: 0.000 to 0.050 mg/L
Nitrite (NO2)
Natural Seawater Value: 0.010 mg/L
Acceptable Range: 0.000 to 0.100 mg/L
Nitrate (NO3)
Natural Seawater Value: 0.050 mg/L
Acceptable Range: 0.000 to 25 mg/L
Phosphate (PO4)
Natural Seawater Value: 0.030 mg/L
Acceptable Range: 0.000 to 0.250 mg/L
Silica (SiO2-3)
Natural Seawater Value: 0.040 mg/L
Acceptable Range: 0.000 to 0.500 mg/L
Potassium (K)
Natural Seawater Value: 390 mg/L
Acceptable Range: 350 to 450 mg/L
Calcium (Ca)
Natural Seawater Value: 400 mg/L
Acceptable Range: 350 to 450 mg/L
Boron (B)
Natural Seawater Value: 4.6 mg/L
Acceptable Range: 3.0 – 6.0 mg/L
Molybdenum (Mo)
Natural Seawater Value: 0.01 mg/L
Acceptable Range: 0.0 to 0.12 mg/L
Strontium (Sr)
Natural Seawater Value: 8.1 mg/L
Acceptable Range: 5.0 to 12.0 mg/L
Magnesium (Mg)
Natural Seawater Value: 1280 mg/L
Acceptable Range: 1100 to 1400 mg/L
Iodine (I¯)
Natural Seawater Value: 0.060 mg/L
Acceptable Range: 0.030 to 0.090 mg/L
Copper (Cu++)
Natural Seawater Value: 0.030 mg/L
Acceptable Range: 0.000 to 0.030 mg/L
Alkalinity (meq/L)
Natural Seawater Value: 2.5 meq/L
Acceptable Range: 2.5 to 5.0 meq/L