Seawater has one of the most stable constitutions, varying only in a few places in its 77% coverage of the earth's surface. This factor makes keeping marines more difficult than freshwater species as the fish involved have little tolerance to changing conditions - they never encounter any - but, advantageously it means that all marine fish will be at home in one common type of water.
Except for those few aquarists who have access to natural tropical seawater, the majority of marine fishkeepers have to rely on preparing their own synthetic variety using commercially-available salt mixes.
To make sure the water is mixed to the correct conditions, test the 'strength' of the mix by measuring the Specific Gravity (S.G) of a sample.
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 example, 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.
It is important to measure the S.G. when the water is at the correct temperature (75oF) and thankfully most hydrometers offered at the aquatic outlets are calibrated at this value.
Synthetic seawater should be mixed in clean plastic containers and aerated for at least 24 hours before use.
One of the curses of the marine aquarium (not helped either by the intense lighting found on most systems) is unwanted algae; this is further encouraged by phosphates and nitrates found in the water used to make the seawater.
Always use a salt-mix that is labelled 'phosphate and nitrate free' and also make sure that water used is also free of these substances too.
Source water can range from absolutely free of anything tapwater (via Reverse Osmosis treatment), nitrate removed by a Nitragon unit or phosphate removed by using RowaPhos(tm) in a filter.
Additionally, clean rainwater can be used.
Nitrate levels can be kept to a minimum im the aquarium by the action of use anaerobic de-nitrifying bacterial colonies within Living Rock, and by regular partial water changes. The build up of nitrate can also be minimised by using a protein skimmer to remove organic substances from the water before the nitrite-forming and nitrate-converting bacteria can get their hands on them.
Keep a check on pH and nitrate levels: rising nitrate levels and a falling pH means it's time for a water change. Whilst nitrate levels in a fish-only collection can be allowed to rise, invertebrates appreciate really low levels (zero, preferably).
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