Dissolved Oxygen In Seawater

October 26, 2008

Don’t worry; we’re not going all scientific. There’s no need to anyway, the detail the marine aquarist needs to know is straightforward.

Oxygen is a very important dissolved gas in the seawater. The livestock need it to survive as we do – no oxygen, no life. If dissolved oxygen is in short supply then the aquarium livestock will be subject to stress, and if the oxygen level is continuously too low disease and maybe death will follow.

It isn’t just the livestock that will suffer either, the bacteria in the bio-filter will too. These bacteria operate the nitrogen cycle when ammonia is converted to nitrite which in turn is converted to nitrate. The ammonia and nitrite are toxins and will kill at quite low levels, so the bacteria’s welfare is paramount. The bacteria that convert the toxins are oxygen hungry and rely on the amount available in the seawater.

In addition if there is a good supply of dissolved oxygen in the seawater the redox potential will be reasonably high meaning a clean environment. For simplicity’s sake the redox potential is a measure of ‘cleanliness’.

Oxygen is taken into the seawater at air/water interfaces, the major one being the aquarium seawater surface with more in the sump, weirs, overflow pipes (where air is also in the pipe) etc. The intake of oxygen is dependent on efficient seawater movement, and without this movement trouble could follow.

There is about twenty times more oxygen in the air than there is in seawater. Seawater in the aquarium should have an oxygen level of between 6 and 8 ppm (parts per million). The average amount for a well designed reef aquarium is 6.5 ppm*. Not a lot really, but quite sufficient if all is operating properly. This amount of oxygen varies somewhat according to the salinity and temperature of the seawater – another reason why high temperatures bring the aquarist closer to the ‘edge’. Further, oxygen levels can fall at night when the aquarium is in darkness as, for example, algae do not photosynthesize. This can be combated by having algae in a sump which has an opposite lighting cycle, that is, the algae are lit when the main display lights are off.

All aquariums, be they coral only, fish and coral or fish only should have adequate circulation and seawater oxygen levels. However, it is the fully stocked fish only system with its higher numbers of fish that could be most at risk.

Fish place the highest demand for oxygen (I do not know the demand for oxygen placed by bacteria) so it follows that the more fish the heavier the oxygen demand. There are two dangers – first, the demand cannot be met because the aquarium is overstocked, and second demand cannot be met because oxygen intake in insufficient.

In the modern aquarium there are some devices that assist with oxygen such as the protein skimmer. Reliance should never be placed on these devices for the purpose of oxygen supply. If they fail there could be trouble. The seawater in the sump, if one is used, should not be counted into the system net gallonage when stocking is being considered so that those gallons assist with seawater quality including oxygen.

So consider a fish only system which is fully stocked and has been stocked correctly. If the system design is good there should not be any problem with oxygen as long as everything is running. Fresh oxygen is being taken in at the air/water interfaces all the time as more seawater reaches these surfaces and is then distributed around the system.

What if a circulation powerhead breaks down? The seawater movement is clearly going to reduce, though there may still be enough oxygen intake – or there may not. In the latter case, there isn’t any adjustment by the fish to economize on oxygen usage, so they will exist on the oxygen that is available. The demand will reduce the oxygen until it has reached critical levels, and the danger of suffocation arises. This of course will be made worse by the demands of the bacteria in the bio-filter. The same applies with a power cut when all circulation ceases. The reduction in the available oxygen could be more rapid and the danger of suffocation would arise more quickly.

It is clear that stocking is an area where great care needs to be taken. In the reef aquarium there is a smaller danger of oxygen problems as seawater quality is protected by having less fish though care still needs to be taken. In a fish only system with its heavier fish load, and not forgetting to consider the higher numbers of oxygen hungry bacteria there will be, the danger of oxygen depletion is higher.

The guidelines for stocking both reef and fish only systems are readily available and should not be exceeded. In addition, for peace of mind and especially in areas where power cuts are known to occur, the aquarist may wish to consider back-up battery operated powerheads, or even a small back-up generator with enough power to drive the aquarium circulation system and heaters.

No aquarist would wish to see the expensively furnished aquarium suffer or even die because of inconsiderate stocking or the event of power loss. The life in the aquarium deserves better than that.


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Record Your Parameters

July 18, 2008

Once the aquarium is up and running then the aquarist should stand back and admire it. This relaxing period is well deserved, after all a lot of time has been spent in planning and obtaining equipment never mind the cost of it all.

When the aquarium is there in total the aquarist is naturally excited. The beautiful fish and/or corals can be mesmerizing. Time passes easily and not just watching the aquarium. Time just passes and seemingly very quickly. In our busy lives one day can blur into the next and so it is with weeks.

Once livestock are in the seawater changes will occur. Bacteria, the foundation of the Nitrogen Cycle are converting wastes. The protein skimmer starts to get busy as dissolved organic matter starts to appear in the seawater. The seawater is going to deteriorate in quality over time, perhaps slowly or perhaps quite quickly. This seawater quality is the number one requirement for success. So what to do?

The first answer, and the correct one, is to be sure and do routine seawater changes. This will dilute unwanted substances and ‘freshen up’ the seawater. Is this enough?

The initial guideline for the amount of seawater to change is 10% weekly. This guideline is intended for new hobbyists who have to start somewhere, and indeed is for anyone as the baseline. As time goes on the aquarist can check if the change amount is insufficient.

This knowledge is obtained by the weekly testing of seawater parameters in the early life of the aquarium (though the testing regime could be modified later it doesn’t stop). Certain tests should be completed routinely once a week at least, such as specific gravity (SG), ammonia, nitrite and nitrate. A further test that is worthwhile is phosphate. In a reef system additional tests could be carried out, for example calcium. These tests will indicate the state of the seawater. Ammonia and nitrite should remain undetectable, but there could be a slow rise in nitrate and phosphate, which is certainly undesirable as they are nuisance algae nutrients. In a reef system calcium is important and this will decline according to livestock demand.

Is the aquarist going to remember all the results of the tests? I don’t think so. The tests take time and to make them really worthwhile they can be used to determine trends. For this they need to be written down.

As said, testing takes time and it is hardly any effort to write down the results. As the weeks pass the aquarist could be reassured that the biological filter is working well as there is never any ammonia or nitrite reading. The rate of nitrate and/or phosphate increase, or lack of it, could help in determining if the amount of seawater being changed is sufficient – or perhaps if the feeding is being overdone. For the reef aquarist, testing the example calcium can determine the amount that is demanded weekly by the corals and enable the aquarist to arrange dosing correctly.

To make the recording of test results really easy why not obtain a small notebook, such as a small lined notebook that children use in schools. Draw columns on the page and head them with the test name, for example ammonia, phosphate, calcium etc. Leave a blank column on the left side so that a date can be inserted for each row of test results.

It will not take a scientist to work out the trends that will be shown. Increases and decreases week by week will be clear. Over a period this will enable the aquarist to take appropriate action such as increasing the seawater change amount, increasing or decreasing the calcium dose etc.

As said seawater quality is the number one requisite for success. The only real way that the aquarist can determine the quality is by testing. Logging the test results is a big move towards easier seawater quality maintenance and at the same time requires hardly any additional time or work.

To assist you we have created some simple maintenance charts which you can download and use if you so desire. These are:

General Notes (Word Document)

Maintenance Chart (PDF)

Reminder Chart (Word Document)

Test Chart (Word Document)


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Step By Step Seawater Change

June 23, 2008

Routine seawater changes are recommended for good reasons, for example minimising nitrate, re-introducing trace elements removed by protein skimming and/or activated carbon, and general freshening up.

The guideline for the amount of seawater to change is 10% of the total system net gallonage. This amount can vary once the aquarist has experience of the system, but routine changes should be done.

So, the seawater change and its preparation then. It is assumed that the aquarist has a seawater safe bucket, dry sea salt, scales, tubes for removing and replacing the seawater (tubes with a diameter of around ¾” (circa 19mm) are best), a heater, and an air pump or powerhead. The heater, air pump or powerhead do not need to be powerful, they are not dealing with a large amount of seawater. There is also a need for a hydrometer.

It is also assumed that the aquarist is aware of the amount in gallons that represents 10% of the system net gallonage.

The best fresh water for use in mixing seawater is reverse osmosis (R/O) and the aquarist will need to ensure that there is sufficient for the mixing process.

First, have a look at the instructions on the sea salt package. It is often stated how much salt is required for a particular gallonage at a particular specific gravity (SG) and temperature.

Routine seawater mixes are going to be made many times in the future so on the first occasion a little extra work is required, which will save time on following mixes.

Using the information from the package, pour an amount of salt on to the scales. Keep it below the suggested amount. Note the amount in a notebook (a notebook is a really good thing to have). Now, pour in sufficient fresh water to reach the required gallonage mark on the bucket. Give the mix an initial stir with a wooden spoon or stick.

Place the heater and powerhead (airstone) into the bucket and plug them in. Put the lid on the bucket and run it for around 8 hours or so (do not seal the lid, drill an air hole if necessary), to ensure the salt is mixed and the temperature is the same as that in the aquarium.

After the requisite period has passed, measure the SG of the seawater. Is it low? If so, add a little more salt, but weigh it on the scales first. Note the weight of the salt with the previous note. Continue with this until the SG is as desired, giving plenty of time for the salt to dissolve, two hours or so. When the required result is achieved, add up the total weight of all the salt used. This is the amount you will need on future occasions. Note the total weight down.

If after the initial period the SG is too high, add small amounts of fresh water until it is correct. When a new mix is to be prepared, reduce the amount of salt used and proceed as above, always keeping a note on salt weight for future use. It is unlikely that the SG will be too high – this is why the salt was reduced from the amount recommended by the manufacturer.

Check the seawater temperature is at the level required. If it isn’t, then the thermostat can be adjusted. Remember that SG is affected by temperature, so check the SG after any temperature adjustment.

The SG and temperature should be the same as those of the aquarium seawater.

Once the process to determine the amount of salt needed per mix is concluded, and the heater brings the temperature to the correct level, future mixes are straightforward.

The appropriate amount of seawater in the aquarium needs to be removed. This is easily achieved with a bucket (separate from the new mix bucket) and a hose. The hose needs to be long enough to reach well into the bucket when the other end is at the bottom of the aquarium.

When seawater is being removed it is a good opportunity to siphon out any debris that is seen. To make this easier by better control of the tube, the aquarist can bind the end of the tube to a length of wood which is stiff but not too wide. The wood can be around 12 to 18″ (circa 305mm to 457mm) long, depending on the depth of the aquarium and the aquarist’s needs.

Place the tube with the wood into the aquarium, then give a good suck on the other end and place that end in the bucket. Seawater will flow through the tube into the bucket. A little practice makes the ’suck’ period easy.

If any debris is noticed it is easily removed so long as the seawater is flowing.

Watch the amount of seawater that is removed and when it has reached the relevant mark on the bucket, remove the wood end from the aquarium to stop the flow. This seawater can be discarded.

Putting the new seawater into the aquarium is really easy. If the aquarium is near some stairs, put the new seawater bucket on the stairs and trail a tube down to the aquarium. It may be necessary to have a different tube for this. It is best if someone can ensure that the tube does not come out of the bucket. At the aquarium end, another one of those technical ’sucks’ will start the seawater flow into the aquarium, Once the bucket is empty the flow will of course cease.

If the aquarium is not near stairs, no matter. The aquarist can use a powerhead (use the one in the new mix) and a suitable diameter tube, and pump the seawater in. The powerhead will not remove all of the seawater, the last bit can be put in with a small jug.

All that remains is to check the SG of the aquarium seawater (with the new seawater in) to ensure all is well. If the SG is a little low, just increase the amount of salt a little on the next new seawater mix. Likewise, if it is a little high decrease the amount.

All that remains is to dry the new seawater bucket and store it until the next time, along with its heater and powerhead or air pump.

One note – when removing the heater from a new seawater mix be sure the heater is cool. If it is not and is out of the water, then the glass could crack.

For the first two or three new seawater mixes check that the SG is as desired, and do an occasional check after this. SG checks of the aquarium seawater itself should be done routinely

Routine seawater changes are simple and quite quick if the above suggestions are followed. Anything that simplifies routine maintenance and gives more time for watching the aquarium can’t be bad.


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Ph In The Marine Aquarium

June 16, 2008

The quality of the seawater in the aquarium is of paramount importance. There are basic parameters that need to be correct and stable, and one of these is pH.

There isn’t going to be any scientific mumbo jumbo here, but the foundations of pH need to be understood. pH is a measurement of the acidity or alkalinity of the seawater. On the pH scale, 7 is neutral, 0 is acid, and 14 is alkaline. (Just for interest, sulphuric acid measures 0, and sodium hydroxide measures 14.)

The pH area that the marine aquarist is interested in is from pH 8.0 to 8.4, so it can be seen that our seawater is on the alkaline side. The often quoted ‘perfect’ pH measurement is 8.3, and aquarists do achieve this. However, if the seawater is not 8.3 there isn’t a need for concern, for successful marine systems can run at the stated 8.0 to 8.4. In fact, a few successful systems run at 7.9. The important part is stability. The pH shouldn’t see-saw greatly as this is detrimental to the livestock.

The life functions of the livestock creates pressure on the stability of the pH. This pressure is pushing the pH towards the acid side of the scale. In the confines of the aquarium this needs to be monitored and controlled.

To find out the pH of the seawater is simple, and requires the purchase of a pH test kit suitable for seawater. These are easily obtainable and should be one of the test kits in use weekly at least.

There is another reason why the pH can fall, and this is because of the day/night cycle. During the day the pH is at the highest point, and at night, when the lights have been off for a few hours, it is at the lowest. This is not always the case – in my aquarium the pH is a constant 8.0, day and night.

Any fall is easily discovered, just do a test after the lights have been on a few hours and after they have been off a few hours. If the fall is slight, there isn’t a problem, but if it is large then action can be taken. Many aquarists use a sump and plant it with the macro algae Caulerpa. The Caulerpa is lit when the main aquarium lights are off and this helps counteract the pH fall.

As has been said, the pH is under pressure to move towards the acidic side of the scale. There is resistance to this pressure by what is termed the buffering capacity of the seawater, or the alkalinity. This alkalinity is mainly because of the carbonate and bicarbonate content of the seawater.

The aquarist should be completing routine seawater changes, as this helps to maintain the alkalinity and thus helps stabilise the pH. However, some aquarists find that they need to protect the pH from falling by additional methods. This is simply done by using a commercially available dry powder, mainly carbonate and bicarbonate, and often called ‘pH Buffer’ or ’Alkalinity Booster’ or similar. The instructions provided should be adhered to. There are other ways which will not be covered here.


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Water Testing – And Not Just The Water In The Aquarium

September 25, 2007

There is an area which I belive at lot of aquarists either do not know they need to do or in some case forget to do.

Aquarists should test the water in the aquarium on a regular basis to ensure that everything is correct and stability is mainteined, however there is another area which needs to be tested periodically.

The water which you have in the aquarium comes from a water source, for the sake of this post I will presume a reverse osmosis unit. Water changes are performed where the ro water is mixed with salt and then added to the aquarium.

However is the water you are mixing with salt ok? [Read more]

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Aquarium pH – What Is It? How Is It Controlled?

July 22, 2007

In a marine aquarium the pH measurement is of great importance. Marine aquarists routinely test their seawater pH level, but what is pH? It is not intended to go into great scientific detail but to give a general explanation of the term. For those who want to delve into the scientific world, I’m afraid you’ll have to look elsewhere. I’ll also give a little information on its control.

First of all, [tag-tec]aquarium pH[/tag-tec] is a measurement of the acidity or alkalinity of our seawater. The measurements are set against a scale which runs from 0 to 14. The centre of the scale, 7, is neutral. 0 is acid and 14 is alkaline. Out of interest, Sulfuric acid measures 0, and Sodium hydroxide measures 14. [Read more]

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Aquarium Water Testing And Parameters

July 9, 2007

Maintaining good water quality in any aquarium, especially a saltwater aquarium is essential. Testing your water parameters is something which should be part of your regular maintenance schedule.

However, although you test for them do you know what each one is for and why it is important – quite a few people don’t and just test the water (which is a good thing) but in my opinion it is also good to understand what each one does. [Read more]

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