Why Am I Burning Through So Much Alk?

[Dkeller_nc]

Good point--I think in my case, 3dKH turned to 4, then 5, then 8 (over a span of a few days), with each rapid fall happening over a period of an hour or two, not a whole day, then a cloud of precipitate on the water's surface ... all just points to what several of the others said. (One interesting thing I saw, if I dosed the carbonate, the exact time it took to complete the whole dose is exactly the time it took for alk to drop back down to where it was, or lower. So if it took me 2 hours to complete the dose, 2 hours later, it would be back down where it was--which led me to the assumption that it was all precipitating out exactly as quickly as I was putting it in.) After swapping, alk has remained steady, consumption is just under 1dKH a day, and pH stays locked on 8.1, even at night.

Jonify - One side note about your observation. It's certainly possible that what you were seeing on the water's surface is precipitated calcium carbonate from a high-pH addition of sodium carbonate. And once calcium carbonate forms, it will not redissolve in seawater to any appreciable extent that a reef keeper would care about. The note here is that the high pH of an alkalinity addition also causes a very temporary precipitation of magnesium in the form of magnesium hydroxide. This is the quickly-clearing "cloud" that reefkeepers that dose by hand have observed. Since magnesium hydroxide is not stable as a solid in seawater unless the pH is about 9.5, you can observe the specifics of the precipitation to determine whether you're forming magnesium hydroxide, calcium carbonate, or both - the Mg(OH)2 will dissipate almost instantly, while a precipitate that lasts more than a few seconds is calcium carbonate.

 

[Jonify]

Perhaps it is just your wording, but I think that statement is a bit misleading. Seawater contains both bicarbonate and carbonate and you have no control over the ratio of the two except via pH. What you add makes little difference to what ends up in the aquarium after it mixes in. The reason one may have some precipitation with higher pH additives is that the local pH is higher, shifting more of the bicarbonate to carbonate locally until it mixes in.

That is what was so frustrating to me--it never seemed to mix back in, even when dripping the dose through a drip acclimation tube, at about 1 drip every 2-3 seconds in front of the power head (which would take about an hour for the full 8 ml dose, representing about 1dKH). I'd test alkalinity and pH and confirm I was at the target, and then a few hours later, it would be like I had never dosed anything at all. With this second tank, there were no fish or coral to consume it that quickly, though I know regular cycling biological processes do use some alkalinity, too.

 

[Randy Holmes-Farley Verified Community Expert]

It is certainly fine to use sodium bicarbonate.

 

[Jonify]

It is certainly fine to use sodium bicarbonate.

A couple of lightbulbs went off in my head tonight while reading your article, What Is that Precipitate in My Reef Aquarium? This is a long response, but observing how many times RHF seems to have to answer the SAME. QUESTIONS. EVERY. DAY. I'm hoping this adds to the body of knowledge for other people googling similar problems. BLUF: In a new reef using salt with elevated levels, calcium may be too high for stable alkalinity adjustments; alkalinity additives dosed too early may cause higher rates of precipitation in these reefs.

It seems that the ratio of calcium in my new reefs compared to alk/pH is too high (looking at your article's figures under "Calcium Carbonate Precipitation: Calcium, Alkalinity, and pH") and as a result is increasing the likelihood of abiotic precipitation. Further, the low organics/lack of a mature bacterial film on the sand/rock/surfaces of both of my newer tanks is exacerbating this. I use RSCP salt (RSCP mixes to 465Ca, 1390Mg, 12dKH, 8.2pH), and in new reefs, this is already starting you dangerously close to calcium carbonate precipitation levels; trying to tweak this too early causes the problem--that is my theory, and I think it will resolve itself once calcium levels naturally drop and my tank becomes more mature.

Here's why I believe this: I filled both new tanks with RSCP salt. After about a week, Alk began to drop, but calcium did not. Calcium should be consumed at about 20ppm to 1dKH of alk consumption. This wasn't the ratio in which it was consumed in my tanks. Calcium would start at 465, alk at 12; alk would drop down to 8, calcium would stay at 465, versus 385, which would be normal consumption. I don't know why this is. (I test alk each day, calcium every week, with Hanna checkers--and I am meticulous about those Hanna checkers and those blasted Ca reagent packets.) So what was happening is, I was trying to drive alk back up to 12dKH with carbonate, but since calcium hadn't dropped previously (and why would it, little/new coral, no coralline yet), the addition of substantially more pH with the carbonate temporarily pushed me past the point of precipitation-likely ratios that RHF shows in his figures, which caused a major precipitation event. This coated surfaces with crystals that the biological coating/magnesium levels could not keep up with/mitigate, which exacerbated every single dose of carbonate after--ending with a white film of precipitation on the water's surface at its highest point. (Some will note clumping of sand with this issue; I did not, since I'm using black sand with very large grain sizes--it's likely harder for precipitate to bind this kind of sand together into clumps.)

Swapping to bicarbonate has rapidly slowed this negative feedback loop down (and might have prevented it entirely, but cannot really know), but calcium in both tanks are still elevated (one is 460, one is 560--remember, the one with 560 is the one on All for Reef, which is dosing both calcium and alkalinity, and which is why Ca has continued to rise in that tank. That tank is not yet consuming Ca and alk at normal levels--alk is in much, much higher demand, either through new cycling processes or through abiotic precipitation or both). I've since stopped All for Reef and put it on just the baking soda so Ca can naturally come back down as coral begin to take it up.

There is still a very, very slight amount of observable abiotic precipitation on surfaces in both tanks several hours after dosing (but nearly unnoticeable with the oldest of the two), even with just the plain baking soda. So, I'm expecting that once calcium drops to below 410, and once the tanks have a more mature biological film over surfaces, I'll be able to dose carbonate without issue, and will be able to increasingly keep alkalinity and calcium levels elevated via dosing, to the same levels of my RSCP salt, without excess precipitation. I suspect more mature reefs do not usually have to deal with this problem. I'll of course keep this updated as time goes by. I'm hoping I will observe something that will be useful to other reefers with completely new tanks who are targeting higher alkalinity levels from the start.

For those who are just reading this message and want to chime in with "why are you running alk that high?" or "have you added a Co2 scrubber to bring up pH versus resorting to alk adjustments?" or "plenty of tanks run FINE at 7.7pH!" read up higher in the thread, I've answered most of those questions

Once I can confirm these assumptions through time and observation, and maybe a few more new tank builds to re-test these assumptions, I will absolutely come back and edit the very first thread to put these tested observations right up top so folks don't have to read through the entire thread.