The provision of correct water conditions at all times is the main secret of success in marine fishkeeping.
Fortunately, salt-mixes are widely available to allow the easy preparation of synthetic sea water that is hard to distinguish from the real thing. When choosing salt mixes it is important to use 'phosphate and nitrate free' types as the inclusion of these two elements will encourage algae growth.
To make sure it is made up to the correct 'saltiness' the SPECIFIC GRAVITY (S.G.) of a sample of the mixture is taken using a HYDROMETER. Always use an aquarium hydrometer and test the water at the correct temperature (approximately 25oC). Most hydrometers offered at the aquatic outlets are calibrated at this value.
In Nature, this approximates to 35 ppt (parts per thousand) or, in practical terms 35 grams of salt per litre of water, approximately S.G. of 1.025; in this case, the S.G of the sample reads 1.024 which is satisfactory for a reef tank but could be lower (1.021-1.023) for a 'fish only' collection.
Synthetic seawater should be mixed in clean plastic containers and aerated for at least 24 hours before use. When filling the aquarium for the first time, the aquarium itself can be used with only buckets being needed for water changes and/or maintenance purposes in the future.
Water used to mix the salts may be already high in nitrates as it comes from the domestic supply; most marinists use water that has been previously treated to remove unwanted elements - this can be done by using 'nitragon' resins or using a Reverse Osmosis (R.O.) unit.
R.O. Water is often available at the marine aquatic retailer for water change purposes and as these are not too large at any one time, the cost is reasonable. Only when filling the tank for the first time is the cost a bit staggering! Alternatively, clean rainwater can be used as source water.
More About Seawater
What Size Aquarium?
Setting-up and Running-In
Fish & Invertebrates
Water Condition Management
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Last updated March 10, 2002