Frequently Asked Questions II

By: Coral Plantations Pty Ltd

Why do pH levels fluctuate in a saltwater aquarium? (and what to do about it )?

 

It is normal in a marine/reef aquarium set-up for the pH to fluctuate, particularly between day and night.

 

 

The ideal pH in a reef system is between 8.2 and 8.5 or between 8.1 and 8.4 depending on what book you read. I prefer the former; ….and should never fall below pH 8.0 or go above pH 8.5 at any time.

 

 

Too high a pH (8.5 +) and too low (8-), reduces calcification by corals and other invertebrates with skeletons. At higher pH calcium, magnesium and probably strontium are precipitated as the solid carbonate forms. At lower pHs there is generally less available calcium and carbonates in the water for the calcification process to occur (by any organism that lays down a calcareous skeleton, and this includes calcareous algae). Natural pH fluctuations occur in the reef system due to photosynthesis activity by organisms using light for growth.

 

During daylight photosynthesis occurs which removes dissolved CO2 from the water (raising the pH) and at night CO2 is released as carbonic acid that reduces the pH. A small variation in pH is said to increase the rate of calcification but significant pH fluctuations have the opposite effects.

 

Several methods exist to reduce the severity of pH fluctuation.

 

The most notable are:

• by increasing the alkalinity/buffering capacity or the water; by adding carbonate hardness to at least dKH8.5 / 150ppm; (using either carbonate , hardness generator, sodium bicarb or Kalkwasser)
• by setting up a reverse lighting system in the sump, filter system or refugium and using algae such as caulerpa, with or without deep sand or mud. This concept also greatly aids in nutrient export if harvested regularly.

 

 

 

It is also not uncommon for pH to gradually fall over time in an enclosed system as a result of accumulated (1) nitrates and (ii) phosphates.

 

 

Accumulated nitrates and organic acids decrease the pH by their acidic nature as well as lowing the alkalinity/buffering

 

 

Accumulated phosphates lower the alkalinity/buffering by precipitating compounds out of solution, in conjunction with calcium and magnesium carbonates.

 

 

One activity that helps to keep the pH high is the denitrification process (by bacteria breaking down the nitrates to gaseous forms). This releases carbonates and bi-carbonates, which in turn increases the alkalinity/buffering. This occurs in anoxic /low-oxygen conditions such as inside live rock, in porous rock-balls and in deep sands beds (especially with a plenum).

 

 

Adding additional carbohydrates into the water aids the denitrification process (e.g. the vodka method, & the Kalkwasser/vinegar method- or calcium acetate).

 

 

Phosphates can be also removed by phosphate absorbing material, the use of Kalkwasser, setting up an algae filtering tank and by using a protein skimmer.

 

My nitrate levels are always about 20ppm; and I cannot get them lower, is that a serious problem?

Once it was said, quite categorically, that nitrate levels should be kept below 5 ppm for good coral and reef life invertebrates health but that fish could tolerate considerably more. Nitrate is the end product of the bacterial “nitrification” process.

 

 

This concept is still true but the maximum “acceptable” nitrate level for invertebrates has been raised considerably in the light of common sense and findings that demonstrate that many beautiful looking reef aquarium systems can exist permanently with nitrate levels of 20 ppm or even slightly higher…….. if other requirements of light, water-flow, chemical levels etc etc are adhered too.

 

 

There are many examples corals thriving in areas close to shorelines; where run-off water carries nitrates (and phosphates) out to sea into the reef system from natural soil erosion and agricultural pollution. Many corals can and do grow faster in higher nitrate regimes.

 

One excellent reference book covers this aspect, and others, in detail and is recommended reading for the keen reefer, namely ”The Reef Aquarium, Volume 3 Science, Art and Technology” by Charles Delbeck and Julian Sprung.

To quote the above authority

The principal disadvantage of higher nitrate levels is the effect nitrification has on alkalinity and pH. The formation of nitrate depletes the alkalinity in a closed system through the release of protons (H+) in the conversion of ammonia to nitrate.

 

 

In addition, nitrate-stimulated growth in corals also causes greater usage(removal) of calcium from the water but can lead to a less dense structure due to rapid growth.

 

 

If high nitrate levels are maintained, it is especially important to observe the calcium and alkalinity levels in the aquarium. Use of calcium and alkalinity effectively counters the depletion of calcium and alkalinity.

 

It has also been proposed that the high nitrate levels might necessitate higher additions of iodine for corals or other creatures that utilise iodine. This hypothesis has not been tested

Where do nitrates come from?

• From any thing living.. broken down by nitrifying bacteria to ammonia and then to nitrite then to nitrate (and this includes dead animals, excess food etc, rotting vegetation etc)
• From water used for water changes and top-ups

 

 

An interesting point made by the above author is:

• Nitrate can also be synthesised by cyanobacteria (red slime mould)- called nitrogen fixation, using a similar mechanism where legume plants fix nitrogen from the atmosphere using nitrogen fixing symbiotic bacteria.

 

How important is carbonate hardness?

• Carbonate hardness generator - contains a spectrum of hardness compounds including sodium bi-carbonate, soda ash and borates. In salt water it is much more effective than sodium bicarbonate or baking soda. It is both cheap and effective, and the effectively keeps pH up .
• Kalkwasser – usually made by dissolving Ca(OH) / hydrated lime in water. The good thing about kalkwasser is that it will supply calcium and as well as keeping the alkalinity level and the pH stable. Often mixed with vinegar for better effects.