Many people who now enjoy reef keeping probably spent vast quantities of money, time and mental anguish trying to reach, and then maintain, the target of perfect water parameters. It is only with experience that you realise perfection may not be obtainable, and the attempts to reach them can be highly de-stabilising to your tank's water chemistry.

pH (acidity / alkalinity) pH 8.2 - 8.3

pH is a logarithmic measure of hydrogen ion concentration, originally defined by Danish biochemist Søren Peter Lauritz Sørensen in 1909. The scale runs from 0 to 14. A Low pH is acidic, a high pH (high hydrogen ions) is alkaline, with a pH of 7 being neutral. Sea water is slightly alkaline, with the perfect pH often being quoted at pH 8.4.

The very animals we keep in the reef tanks work against us maintaining a perfect pH, producing CO2 (an acidic oxide that reacts with water to give carbonic acid ) and waste products that release organic acids. These acids soak up hydrogen ions, lowering pH.

In the vastness of the sea, organic acids and CO2 are easily absorbed / diluted, and as a result, the pH is incredibly stable. The same cannot be said for the relatively small and closed system of your reef tank. The tank's pH will usually rise whilst the main lights are on, then fall slowly when they are turned off. This regular rise and fall can be called the pH cycle. This cycling affect is detrimental to most marine life as it has not evolved to cope with changing pH levels. To prevent problems we try to minimise the change to a 0.2 drop in pH or less.

The ability to resist change in the pH value in a tank depends upon the buffering capacity of the water. This is the ability of the water to 'soak' or use up organic acids. One of the main factors that affects the tank's buffering ability is Alkalinity levels (this is discussed later)

Even with good buffering, it is not realistic to expect to achieve the same pH stability and level as the sea. Although I have noticed a form of self perpetuating peer pressure, which has the result of making the inexperienced try for the impossible.

I have often heard customers in shops boasting of a pH at over pH 8.4 and Calcium levels of well over 500 ppm (parts per million) . On one memorable occasion I heard a customer telling someone with great pride that he struggled to keep pH below 9. I wonder if anything was left alive in the tank? In truth, a pH over 8.5, and supersaturated calcium levels are nothing to boast about, as calcium in the water will be precipitating out of solution, resulting in the pH and calcium levels crashing as well as any internal pumps calcifying due to the precipitation.

To try to counter this weird peer pressure, I can say that in my experience pH rarely stabilizes (at the high point of the pH cycle) at more than 8.3 and can settle, with no adverse effects, as low as pH 8.2.

Where the pH value of your tank settles depends greatly on the type of system you implement to maintain calcium levels.

Calcium Levels, 425 - 450 parts per million

Corals need calcium to grow their skeletons (soft and hard corals). If the level is too low (below 250ppm) corals will stop growing and in extreme cases hard corals will shed their flesh. Natural sea water has about 425 to 440 parts per million of calcium. This is perfectly adequate, trying to increase levels far beyond this results in supersaturation and precipitation of calcium back out of solution (usually in the submerged pumps)

There are many methods used for adding calcium to the reef tank. A fair few involve spending vast amounts of money on chemicals, good for the industry, bad for the pocket. I have chosen therefore to only discuss two methods that neatly tie into pH control, and in the long run can be a low cost solution. These are kalkwasser solution, also called lime water which is a saturated solution of calcium hydroxide (you can buy food grade calcium hydroxide cheaply) and a carbon dioxide (CO2) fed calcium reactor.

A calcium reactor slowly dissolves a source of calcium carbonate e.g. clean coral sand (dirt cheap) in a reactor chamber containing tank water acidified by the controlled addition of CO2 (dirt cheap) down to a level of pH 6.4. This water slowly drips into your tank, adding calcium. The side effect is that the addition of this acidic water may reduce you tank's pH by 0.1 in normal operation.

On the other hand, the use of kalkwasser (calcium hydroxide) via a stirrer / reactor on the freshwater top-up or just adding it via a dripping bag can cause the pH to rise much more than a mere 0.1pH . This is because lime water solution, when fully saturated, has lots of hyroxide ions, and as a result is very alkaline with a pH of 12 .5. This very high pH means that the lime water solution. needs to be added very slowly, to avoid at all costs the pH rising above 8.5 when calcium and other elements will begin precipitating from solution.

At this point you are probably thinking the affects on the pH levels of my two chosen methods of calcium addition are highly undesirable, but wait a minute. Your tank already has a pH cycle, which you need to minimise. So why not use these methods' effects on pH levels to your advantage?

Kalkwasser (calcium hydroxide / limewater) can be added slowly after the lights go out, to top up evaporated water, counteracting the tendency for the pH to drop. Whereas the calcium reactor can be used during the day when pH naturally rises. The end result is a far more stable pH level

Alkalinity 7 - 9 dKH

Alkalinity is needed to buffer possible pH changes. Natural sea water has a level of 7 dKH. However as we do not have the advantage of the sea's massive dilution potential to deal with excess organic acids and CO2, we need a slightly higher alkalinity of about 8-9 dKH to successfully buffer a tank's pH.

Why not go for a really high alkalinity level? Well, very high levels of over 11dKh prevent some corals (usually soft) from being able to take up calcium.

Using a combination of a calcium reactor and kalkwasser gives you an easy method for controlling the tank's alkalinity. Calcium reactors increase alkalinity, in fact if used 24 hours a day they can raise the level too high. In contrast, kalkwasser tends to lower alkalinity. When used as described above (calcium reactor in the day, kalkwasser at night) they tend to maintain a good alkalinity level. The alkalinity level can be adjusted by fine tuning the periods that the calcium reactor and kalkwasser are used for

Magnesium 1200 - 1400 ppm

Often overlooked, this is a very important element for providing a stable water chemistry. It is needed at levels over 1200 parts per million (ppm) to hold calcium in solution at the desired levels. However, as usual you can have to much of a good thing, at levels over 1500ppm you will start to lose calcium out of solution. In practice, my calcium reactor maintains the level for me (the dissolving coral contains magnesium) and only an occasional top up is required (another benefit of a calcium reactor). When adding Mg it is much cheaper to use a crystal form of additive.

Temperature

In my opinion there are three important things to remember about the temperature of a reef tank

1. Never let the temperature exceed 29 degrees celcius, otherwise your corals may bleach. A process where, as the name suggests, the corals lose their colouration. This happens because the coral expels a symbiotic algae from its tissues due to stress. These algae provide colour directly and by triggering the production of often brightly coloured ultra violet filtering chemicals. The loss of the food that was provided by the expelled algae often causes the coral to slowly starve to death as a result.
2. It should be reasonably stable, try to keep any fluctuation to within a couple of degrees celcius in a 24 hour period. The sea is a massive body of water (no news here then) and as such, temperatures on a reef are remarkably stable. As a result, reef life has not evolved to handle rapid temperature changes.
3. Never let it fall below 23 degrees celcius. The animals used to live on a reef, not in the arctic! Many chemical reactions necessary for life are temperature critical, honed to the environment the animal has evolved in. If the animal's temperature drops too far below its norm, the chemical reactions can't happen and the animal dies.

Keeping the above in mind, the actual temperature you choose to run your tank at is personal choice. When I first started keeping a reef tank I was told to make sure the tank did not exceed a temperature of 24 degrees celcius, and did not fluctuate by more than 1 degree celcius. Rather a tall order, and not one I suggest you try to duplicate.

These days I keep my tank between 25 - 27 degrees celcius. This has had the affect of speeding up the coral growth (compared to 24 degrees celcius) and giving me a buffer zone at both ends of the temperature spectrum, should something go amiss.

The method you choose to achieve temperature control is largely personal choice. Fans are reasonably effective and cheap but without soundproofing can proove noisy. Chiller units are expensive to buy and run, but extremely effective (if large enough for the job) and highly controllable. Unfortunately they are also noisy. For inspiration you can have a look at the design we used for cooling our tank and keeping the noise down here.

Nitrite, Nitrate and Phosphate The desired levels are set out below in the conclusion table

Sumary

Like many things in life, the key to a happy tank is stability. Corals and fish will, in my experience, happily accept water parameters a little off the norm, as long as they remain fairly stable and are within a safe margin. So ignore that peer pressure and relax. Below is a table with my take on water parameters

Calcium	Magnesium	Ph	Alkalinity dKH
Target : 450ppm (often boasted at 500 ppm plus)In practice 425ppm is perfectly adequate. You can experience problems with hard corals at levels below 250ppm	Target: 1450ppm In practice try to keep it over 1200ppm Magnesium is needed to prevent Calcium from precipitating out of solution and the pH crashing	QuotedTarget: pH 8.4 In practice pH 8.3, one very simple tank I had stabilised at about pH 8.0 longterm. It had no problems! Note that alkalinity and magnesium levels were high	Target: 8 dKH Important for the buffering capacity of the water (ability to maintain a stable pH) In practice I try to keep this at least at 8dKH but below 10 dKH

Temperature	Nitrite	Nitrate	Phosphates
Quoted Target: 24 degrees CentigradeNEVER above 29 C In practice corals often grow faster at about 25 - 27C Stability is important. I try to keep within 2C per 24 hour period	Target: UndetectableToxic waste product - should only be detectable during initial tank set up (tank cycling) In practice once cycled you should not detect this chemical	Quoted Target: As low as possible to help prevent Algae In practice Can be as high as 20ppm with no problems as long as phosphates are low	Target: Undetectable In practice must be as low as possible to prevent Algae growth and allow hard corals to grow