Ideal alkalinity?

[GillMeister]

I'm looking for some unscientific opinions about what the ideal alkalinity is for healthy SPS corals. I'm running a little on the low side right now, at about 8.0. For those that have had time to try different alkalinity levels what has worked best for you?

 

[Gtinnel]

I try to keep mine around 8 because I have fairly low nutrient levels. I'm trying to avoid burnt tips.

 

[Randy Holmes-Farley]

Here's the alk blurb from one of my articles. The bolded section relates to your question.

Alkalinity

Like calcium, many corals also use "alkalinity" to form their skeletons, which are composed primarily of calcium carbonate. It is generally believed that corals take up bicarbonate, convert it into carbonate, and then use that carbonate to form calcium carbonate skeletons. That conversion process is shown as:

HCO3- → CO3-- + H+

Bicarbonate → Carbonate + proton (which is released from the coral)

To ensure that corals have an adequate supply of bicarbonate for calcification, aquarists could just measure bicarbonate directly. Designing a test kit for bicarbonate, however, is somewhat more complicated than for alkalinity. Consequently, the use of alkalinity as a surrogate measure for bicarbonate is deeply entrenched in the reef aquarium hobby.

So, what is alkalinity? Alkalinity in a marine aquarium is simply a measure of the amount of acid (H+) required to reduce the pH to about 4.5, where all bicarbonate is converted into carbonic acid as follows:

HCO3- + H+ → H2CO3

The amount of acid needed is equal to the amount of bicarbonate present, so when performing an alkalinity titration with a test kit, you are “counting†the number of bicarbonate ions present. It is not, however, quite that simple since some other ions also take up acid during the titration. Both borate and carbonate also contribute to the measurement of alkalinity, but the bicarbonate dominates these other ions since they are generally lower in concentration than bicarbonate. So knowing the total alkalinity is akin to, but not exactly the same as, knowing how much bicarbonate is available to corals. In any case, total alkalinity is the standard that aquarists use for this purpose.

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.

 

[arking_mark]

I've been going down this rabbit hole. Apparently, there is a defined relationship between dissolved CO2, Alk, and pH. Given any two of these numbers, you can calculate the 3rd. However, digging deeper, it really is more complicated than that. I like this chart @Randy Holmes-Farley has in one of his articles and was using it as a rough target for my tank. My pH is locked in at 8.3 (+/- 0.02). So I am targeting 2.75meq/l or 7.7dKHish. Not a hard target, but I know I shouldn't naturally be at 9 or 7.

 

[MichaelReefer]

I'm looking for some unscientific opinions about what the ideal alkalinity is for healthy SPS corals. I'm running a little on the low side right now, at about 8.0. For those that have had time to try different alkalinity levels what has worked best for you?

I am running 9.0

 

[Pntbll687]

I'm trying to keep mine at 9.0 as well. Gives little buffer in either direction.

 

[malacoda]

My tank is mixed reef with roughly 60% SPS and 40% LPS.

I used to keep my alk at 9-10 and it would occasionally touch on 11. Most corals were fine ... but I noticed my alveopora became unhappy above 10.

So, out of curiosity, I switched to gradually brought my it down 7 (nearer to NSW) to see what happened...

I still see good growth from everything. Some SPS actually grow a little faster, some a little slower, but overall, still nice growth. And my alveopora are much happier and growing much faster.

For added info...

When running an alk of 9-10, my PO4 was 0.02 and NO3 was 1-2 (considered by some to be a bit low for that level of alk).

With my alk currently at about 7.5, my PO4 is about 0.12 and my NO3 is 0 (according to my most recent ATI ICP test).

 

[arking_mark]

My tank is mixed reef with roughly 60% SPS and 40% LPS.

I used to keep my alk at 9-10 and it would occasionally touch on 11. Most corals were fine ... but I noticed my alveopora became unhappy above 10.

So, out of curiosity, I switched to gradually brought my it down 7 (nearer to NSW) to see what happened...

I still see good growth from everything. Some SPS actually grow a little faster, some a little slower, but overall, still nice growth. And my alveopora are much happier and growing much faster.

For added info...

When running an alk of 9-10, my PO4 was 0.02 and NO3 was 1-2 (considered by some to be a bit low for that level of alk).

With my alk currently at about 7.5, my PO4 is about 0.12 and my NO3 is 0 (according to my most recent ATI ICP test).

What are your typical pH levels?

 

[MarshallB]

Since my home is so well insulated I run my alk ~ 9.5-10. This gives me an average of 8.2 PH. you can see below I still get pretty major swings even with using a fuge at night. Without dosing I was getting PH dips to mid 7's.

Tank is 125 gallon. I have a lot of frags but they are still pretty small so my alk uptake isn't giant yet. Im dosing 11ml a day to keep alk consistent.

 

[arking_mark]

Since my home is so well insulated I run my alk ~ 9.5-10. This gives me an average of 8.2 PH. you can see below I still get pretty major swings even with using a fuge at night. Without dosing I was getting PH dips to mid 7's.

Tank is 125 gallon. I have a lot of frags but they are still pretty small so my alk uptake isn't giant yet. Im dosing 11ml a day to keep alk consistent.

Are you dosing a high-ph Alk supplement? Using a CO2 scrubber? Outdoor air plumber to your skimmer?

 

[GillMeister]

I had high alkalinity for a while, over 12.0, and found that my corals weren't as happy. Randy's indicating lower alk might be better and that's where I am. I also have low nutrients which also means I should stay at 8.0. I've been trying to maintain some stability using ATI Essentials Pro and the results have been wonderful. I do have issues with pH fluctuations of 0.2 per day but I don't see anyway of controlling this without creating other problems.

This is a lot of great feedback so far, guys!

 

[GillMeister]

I've been going down this rabbit hole. Apparently, there is a defined relationship between dissolved CO2, Alk, and pH. Given any two of these numbers, you can calculate the 3rd. However, digging deeper, it really is more complicated than that. I like this chart @Randy Holmes-Farley has in one of his articles and was using it as a rough target for my tank. My pH is locked in at 8.3 (+/- 0.02). So I am targeting 2.75meq/l or 7.7dKHish. Not a hard target, but I know I shouldn't naturally be at 9 or 7.

You're maintaining pH +/- 0.02? What's your secret? My swings are an order of magnitude higher than this and nothing I do changes it. I even have chaeto with lights on after the DT goes dark.

 

[malacoda]

What are your typical pH levels?

They were about 7.8 - 8.3 (night to day).

A couple months ago I ran a line from my skimmer to outside air. Since then, they have been 8.0 - 8.4. Measured via Salifert.

 

[arking_mark]

They were about 7.8 - 8.3 (night to day).

A couple months ago I ran a line from my skimmer to outside air. Since then, they have been 8.0 - 8.4. Measured via Salifert.

Excellent.