Inconsistency of ideal suggested alkalinity levels.

[TWYOUNG]

I've always seen 8-12 as the suggested goal for reef aquaria alkalinity and thus aimed for 10 since it's right in the middle allowing some movement in either direction. Every type of ICP test I have done always tells me anything above around 8.5 is above their recommended level resulting in a caution to lower it. Can anyone please explain this discrepancy to me?

 

[Randy Holmes-Farley]

I've always seen 8-12 as the suggested goal for reef aquaria alkalinity and thus aimed for 10 since it's right in the middle allowing some movement in either direction. Every type of ICP test I have done always tells me anything above around 8.5 is above their recommended level resulting in a caution to lower it. Can anyone please explain this discrepancy to me?

I’ve been recommending 7-11 dKH for more than 20 years, but each person has their own recommendations.

Heres my commentary from an article:

Unlike the calcium concentration, it is widely believed that certain organisms calcify more quickly at alkalinity levels higher than those in normal seawater. This result has also been demonstrated in the scientific literature, which has shown that adding bicarbonate to seawater increases the rate of calcification in some corals. Uptake of bicarbonate can consequently become rate limiting in many corals. This may be partly due to the fact that the external bicarbonate concentration is not large to begin with (relative to, for example, the calcium concentration, which is effectively about 5 times higher).

For these reasons, alkalinity maintenance is a critical aspect of coral reef aquarium husbandry. In the absence of supplementation, alkalinity will rapidly drop as corals use up much of what is present in seawater. Water changes are not usually sufficient to maintain alkalinity unless there is very little calcification taking place. Most reef aquarists try to maintain alkalinity at levels at or slightly above those of normal seawater, although exactly what levels different aquarists target depends a bit on the goals of their aquaria.

Interestingly, because some corals may calcify faster at higher alkalinity levels, and because the abiotic (nonbiological) precipitation of calcium carbonate on heaters and pumps also rises as alkalinity rises, the demand for alkalinity (and calcium) rises as the alkalinity rises. So an aquarist generally must dose more calcium and alkalinity EVERY DAY to maintain a higher alkalinity (say, 11 dKH) than to maintain 7 dKH. It is not just a one-time boost that is needed to make up that difference. In fact, calcification gets so slow as the alkalinity drops below 6 dKH that reef aquaria rarely get much below that point, even with no dosing: natural calcification has nearly stopped at that level.

In general, I suggest that aquarists maintain alkalinity between about 7-11 dKH (2.5 and 4 meq/L; 125-200 ppm CaCO3 equivalents). Many aquarists growing SPS corals and using Ultra Low Nutrient Systems (ULNS) have found that the corals suffer from “burnt tips†if the alkalinity is too high or changes too much. It is not at all clear why this is the case, but such aquaria are better served by alkalinity in the 7-8 dKH range.
As mentioned above, alkalinity levels above those in natural seawater increase the abiotic precipitation of calcium carbonate on warm objects such as heaters and pump impellers, or sometimes even in sand beds. This precipitation not only wastes calcium and alkalinity that aquarists are carefully adding, but it also increases equipment maintenance requirements and can “damage†a sand bed, hardening it into a chunk of limestone. When elevated alkalinity is driving this precipitation, it can also depress the calcium level. An excessively high alkalinity level can therefore create undesirable consequences.

I suggest that aquarists use a balanced calcium and alkalinity additive system of some sort for routine maintenance. The most popular of these balanced methods include limewater (kalkwasser), calcium carbonate/carbon dioxide reactors, and the two-part/three part additive systems.

For rapid alkalinity corrections, aquarists can simply use baking soda (sodium bicarbonate) or washing soda (sodium carbonate; baked baking soda) to good effect. The latter raises pH as well as alkalinity while the former has a very small pH lowering effect. Mixtures can also be used, and are what many hobby chemical supply companies sell as “buffersâ€. Most often, sodium carbonate is preferred, however, since most tanks can be helped by a pH boost.

 

[gbroadbridge]

I've always seen 8-12 as the suggested goal for reef aquaria alkalinity and thus aimed for 10 since it's right in the middle allowing some movement in either direction. Every type of ICP test I have done always tells me anything above around 8.5 is above their recommended level resulting in a caution to lower it. Can anyone please explain this discrepancy to me?

Natural Sea water is around 7.5. It is very stable because the ocean is vast and consumption is replaced.

In a tank we tend to keep it high enough so that should coral consumption cause a drop before we correct it, it will not drop to below around 5 where it can cause problems to coral.

So I keep mine around 8 which gives a bit of wiggle room.

If you run it higher the coral can grow skeleton faster than it can grow skin to cover the bone if you have insufficient nutrients.