We all know seawater. It covers the majority of the earth’s surface and can be measured in cubic miles. There’s definitely a lot of it and it doesn’t just contain salt, it is a mix of many things.

Tropical fish and corals are supplied with all they need by the seas and oceans. Within the natural seawater mix are all they need to grow and remain healthy. Fresh seawater is constantly washing the reef and there is never a shortage to cause trouble. There are around 34 grams of salts per liter dissolved in seawater, and this isn’t just the table salt (sodium chloride) that we use. There are around 13 major elements, 64 trace elements and around 10 others in very small amounts. All the elements found on land are there, as over the millennia they have been washed by rain into the rivers and then transported to the sea. The sea continues to become more salty but at a very slow rate. This rate is so slow the sea is considered stable, with the reef area specific gravity (SG) considered to be 1.026 (salinity in parts per thousand will not be used as SG is the usual measurement for the hobby).

The wild reefs then are stable, but what of our aquariums? An aquarist with a very large aquarium has only to stand by the sea and mentally put their aquarium in it to realize how tiny their aquarium is. How can fish and corals survive in such a habitat? The answer is a mixture of human technology and nature’s assistance: technology such as heaters and protein skimmers, plus nature’s assistance such as biological filtration, where a life support system is provided by bacteria.

There are very high quality dry salt mixes available nowadays, in which the manufacturers have incorporated the majority of the dissolved salts in the sea, and in more or less the correct amounts. This is definitely an achievement and a boon to the aquarist.

The aquarist, with the aquarium system complete, has only to mix the dry salt with fresh water and ensure the SG and temperature are correct, then mature the biological filter. When the filter is mature the livestock slowly go in and all is done. It’s as easy as that? Unfortunately it isn’t. As time passes, the fact that life is present in the aquarium seawater means that the quality of the seawater will reduce. The success of the aquarium depends on the quality of the seawater to a very large extent, and part of this quality is its stability.

Let’s have a look at what could happen to the seawater.

The specific gravity could change because freshwater evaporates. If the sea salt has been mixed to the required SG, topping up evaporation with seawater will cause the SG to increase. Salt creep which creates the dried salt deposits that occur on glass and wires etc above the seawater surface, could cause in time a drop in SG. The aquarist needs to top up with freshwater to counter evaporation.

A check with a hydrometer or a refractometer should be made at least weekly to ensure the SG is at the correct level. A fish only aquarium could have an SG of 1.022, though a reef system should have a minimum SG of 1.024. Never add dry salt directly to the aquarium (except for the initial fill), it is better to adjust the SG of the seawater destined for a routine water change.

Another important measurement that needs to be checked is pH. This is an acidity/alkalinity measurement. pH is measured on a scale of 0 to 14, 0 being acidic (sulfuric acid) and 14 being alkaline (sodium hydroxide). 7 is neutral. Marine life can be healthily maintained in an aquarium at a pH from 8.0 to 8.4. This does not mean that they are all ‘perfect’, the measurement often given for this is 8.3, and this measurement has some advantages. If the measurement is in the range given it is acceptable provided it is stable. A badly fluctuating pH is undesirable. A check of the measurement should be made at least once weekly using a marine grade pH test kit. If the pH is found to fall noticeably the aquarist can counteract this by adding a buffer, usually called a ‘pH buffer’ or ’booster’ or the like. The manufacturer’s instructions must be followed and a check made into why there is a fall in pH. The reason could be overfeeding, overstocking or a lack of routine water changing (see below). pH can naturally fluctuate giving a different reading during darkness and daylight. A small fluctuation is acceptable.

There are two measurements that apply to all aquarium types, and these are ammonia and nitrite. In the bio-filter maturing period these are measured very regularly as they indicate the activity of the filter. When maturity has been achieved, regular checks continue twice weekly or more during the unstable early days of the aquarium when stocking is in progress. When the aquarium is fully stocked the checks can be reduced to once weekly provided all is well. All the checks are done with ammonia or nitrite test kits. There is only one good reading for ammonia and nitrite and that is ‘zero’ or perhaps it is better put as ‘undetectable.’ Why? Ammonia and nitrite are toxic and deadly to marine life, ammonia being a little more toxic. If there is a reading for either, find out why. Are the livestock overfed, is the aquarium overstocked, is there a problem with the bio-filtration?

So those are the four essential basic measurements that apply to any aquarium type: SG, pH, ammonia and nitrite.

It doesn’t end there. The life in the aquarium needs to feed and of course carries on with normal life cycles. This can create further checks which are important to quality. Ammonia and nitrite have been mentioned, and these two are the first in a chain generally known as the ‘Nitrogen Cycle’. This cycle is created by the bacteria in the bio-filter: they convert ammonia to nitrite and nitrite to nitrate. Nitrate is not toxic in the same terms as ammonia and nitrite but is detrimental to the seawater quality at elevated levels. In a reef aquarium nitrate should be kept below 10ppm. This is more difficult in a fish only system with its higher fish load and subsequent feeding, but the level should be kept as low as possible. When the aquarium is matured and stocking is in progress a check of nitrate levels should be made using a nitrate test kit, and this test should be continued throughout the aquarium’s life though the frequency can be reduced according to experience. Note that nitrate is a known nuisance algae nutrient, see below.

Just one more similar to nitrate and this time its phosphate. Phosphate gets into the aquarium mainly through the food supplied to the livestock. It can cause problems with hard coral growth but the main reason it is undesirable is that it is a known nuisance algae nutrient. Nuisance algae is enough in some cases to cause an aquarist to want to tear their hair out or even give up the hobby. Test the seawater with a phosphate test kit, it’s worth it! The seawater level should be 0.03ppm or less, preferably undetectable.

For all aquarium types, in addition to the four basic tests already mentioned, nitrate and phosphate are worthy additional tests that can be made weekly then the period reduced if all is well and the trends within the seawater known.

The basic tests above could well be sufficient for a fish only aquarium, but a reef aquarium needs more. This is because of the presence of corals etc. The corals make demands on the content of the seawater and one of the major demands is for calcium, particularly with ‘hard’ corals. The amount in the seawater can be discovered by using a calcium test kit. The calcium level should be maintained around or above 420 parts per million (ppm) though keeping the level absolutely stable is not necessary. Calcium should not be allowed to fall below 400ppm. The calcium is needed with hard corals because the corals use it to produce calcium carbonate for their growth. Soft corals require it as they have ‘spicules’ in their tissues which I believe assist with stiffness when they are expanded. If the reef aquarium is reasonably small the aquarist can supplement calcium by purchasing commercial solutions. If this is done it follows that the mixing/use instructions must be carefully followed.

For larger aquariums supplementation could be uneconomical with packages, and would normally be better with, for example a calcium reactor. This is a device that runs seawater through a calcium rich media in acidic conditions, causing the media to dissolve. The outgoing seawater is dripped into the aquarium thus supplying calcium. A calcium test kit must be used so that levels are monitored. The calcium demand of the aquarium will become known and measures can be taken to ensure sufficient but not over supply. There are other ways of providing calcium.

In a reef aquarium (and in a fish only system if pH is a problem) it is important that alkalinity is at a desirable level. This is measured using an alkalinity test kit. Alkalinity tests measure in ppm, meq/l or dKH. The measures are the same as are kilometers and miles which give different numbers for the same distance. Alkalinity is often called ‘buffering capacity’ which is more descriptive, or again it can be called ‘carbonate hardness.’ All the titles come to the same thing. The life actions of the livestock, feeding etc are constantly putting pressure on the pH to fall towards the acidic side of the scale. Without this buffering capacity, the pH could fall and might do so to such an extent that it would be very detrimental to livestock. The buffering capacity is provided mainly by the carbonate and bicarbonate content of the seawater. Acids in the seawater are ‘bound’ by the carbonates/bicarbonates and therefore the pH does not fall. If the buffering capacity is found to fall significantly then supplements can be commercially obtained, often called ‘pH booster’ or ‘alkalinity booster’, or something similar. Again the manufacturer’s instructions must be followed. Keeping alkalinity a little higher than that in natural seawater is considered beneficial. The level should be maintained at 125 to 200ppm, which is 7 to 11dKH, which is 2.5 to 4meq/l.

Seawater can become loaded with dissolved organic matter (DOM) which is generally undesirable, though studies have suggested that corals can make use of it as food by absorption to an extent. Excessive DOM can sometimes be seen as a bubble scum on the surface of the seawater. The answer to this is very simple. Manufacturers provide a device called a ‘protein skimmer’. They are available in ‘hang-on’ and ‘stand alone’ types so there is usually a model to suit any aquarium. The protein skimmer is regarded as a ‘must’ for an aquarium system of any type. It operates by pumping seawater through a chamber full of tiny air bubbles. The DOM molecules have a water loving ‘top’ and a water hating ‘bottom’. This is why DOM could accumulate at the seawater surface, the ‘bottoms’ are attracted to the air/water interface. When the seawater passes through the bubble chamber of the protein skimmer, the DOM molecule ‘bottoms’ are attracted to the bubbles and attach to them, many to a bubble. The protein skimmer is designed so that the bubbles slowly rise and enter a throat which leads to a cup. Clean bubbles collapse, but those with molecules attached are stiff and fall into the collection cup which can be emptied from time to time. The colour of this effluent is brown to very dark and it can also be smelly. When choosing a protein skimmer, the general guideline is to get one that will handle twice the net gallonage of the total aquarium system, to take account of any over claim of performance by the manufacturer. A skimmer oversized this way will not be detrimental to the system. A good protein skimmer is a very worthwhile investment. There is one negative and that is the skimmer will remove trace elements that are better left in the seawater. In view of the benefits of a skimmer this is ignored. If desired, the aquarist can supplement with trace elements, though routine seawater changes (see below) will replace at least partially those lost.

An action that should be taken to ensure high seawater quality is to complete routine seawater changes. This is done weekly and the guideline amount to change is 10% of the total system net gallonage. The period and percentage can be varied as the aquarist’s experience grows though the changes should be done. The advantages of completing these seawater changes are that the points mentioned above all benefit: any nitrates and phosphates present are diluted, DOM in the seawater is reduced, trace elements removed by protein skimming are replenished though perhaps partially, alkalinity is improved, pH is to an extent protected and SG is enhanced. It is important that the seawater going into the aquarium is at the correct temperature and SG, and before it goes in it should have been left to mix and heat at least overnight.

There are a few requisites that are essential to a successful marine aquarium, and one of them at the top of the list is seawater quality. If the aquarist gives time to some simple testing and seawater changing, and also has regard to some additional parameters for a reef aquarium, the reward is a very much enhanced probability of a successful and beautiful display.