Basic coral reaction to chemistry questions

[Idech]

My tank is 3 years old + but has been experiencing a lot of change during that time due to Dino battles. Now for the past 2 months, I have been working on keeping it stable for the first time and observing the effect on coral growth (I could not keep most corals before).

I now do small daily WC and use a dosing pump for ESV Ionic ca and ALK and 2 ml Bacto Energy per day (nitrates used to be over 90 ppm). I run Phosbond to bring down phosphates (used to be over 1 ppm).

Parameters :
PH : 8.0
Mag : 1320
Cal : 430
Salinity : 1.026
Phosphates : 0.12
Nitrates : 15
Alk : 7.9

I have been observing that when phosphates climb a bit, the tank uses less alkalinity. For example, my Phosbond needed to be changed and the phosphate climbed from 0.12 ppm to 0.17 ppm. Then within a day or two, the alk increased from 7.9 to 8.3-8.4. I changed the Phosbond and the next day, the alkalinity was down to 8.1 and tomorrow morning it should be back to 7.9.

My questions :

1) why is there such a big impact on corals when phosphates rise (they consume less alk and they don’t grow) in my tank ? I see so many beautiful tanks when people say they don’t even test or that their phosphate levels are a lot higher ?

2) How do experienced reefers manage this alkalinity/phosphate relation ?

Thanks !

 

[Randy Holmes-Farley Verified Community Expert]

I actually do not think the alk change you saw was most likely from the phosphate rise. It is likely true that phosphate reduces calcification, both abiotic and by corals, but I don’t think the effect of the small phosphate change you saw causes that large of an alk demand reduction.

There may be many explanations, including other things the phosbond was binding, including organic toxins, or a dosing pump change, or test variability.

 

[Idech]

I actually do not think the alk change you saw was most likely from the phosphate rise. It is likely true that phosphate reduces calcification, both abiotic and by corals, but I don’t think the effect of the small phosphate change you saw causes that large of an alk demand reduction.

There may be many explanations, including other things the phosbond was binding, including organic toxins, or a dosing pump change, or test variability.

I’ve witnessed it at least 3 times now.

 

[Randy Holmes-Farley Verified Community Expert]

Do you have any cyano or dinos that might be producing toxins?

 

[Randy Holmes-Farley Verified Community Expert]

I’d also add that it is known that GFO (part of phosbond) induces precipitation of calcium carbonate on it and downstream of it, likely due to iron release. That added alk demand will reduce as the material gets well coated with organics, phosphate, bacteria, and trace elements.

 

[BryanM]

I cannot explain your observations.

I can tell you that my tank has run phosphate levels for months between .6-.9, actively trying to lower them, but no ill effects on corals that I could every really point to that as the cause.

In other words I think something else is going on... And when I went through my dinos period I definitely lost a lot of corals.

 

[Idech]

Do you have any cyano or dinos that might be producing toxins?

I had Dino’s for all the life of this tank (3+ years) but they have been managed for 3 months now. I think I have a bit of cyano on the rocks but nothing concerning.

I’d also add that it is known that GFO (part of phosbond) induces precipitation of calcium carbonate on it and downstream of it, likely due to iron release. That added alk demand will reduce as the material gets well coated with organics, phosphate, bacteria, and trace elements.

The alk tested higher after my Phosbond was depleted and the phosphates had increased. Does it make corroborated that fact ?

I cannot explain your observations.

I can tell you that my tank has run phosphate levels for months between .6-.9, actively trying to lower them, but no ill effects on corals that I could every really point to that as the cause.

In other words I think something else is going on... And when I went through my dinos period I definitely lost a lot of corals.

Oh I couldn’t keep any corals or anemones for 5 years (even with my previous tank I had them) so I know the feeling.

I regularly test on the microscope and I did it again yesterday and saw no Dino’s. I did remove most of my sand recently so it helps.

I did see a lot of diatoms but this is normal since a lot of si,I te was dosed. Would diatoms be the culprit ?

 

[BryanM]

I did see a lot of diatoms but this is normal since a lot of si,I te was dosed. Would diatoms be the culprit ?

Would diatoms be the culprit of what? They feed off silicates and nutrients, so they would actively lower phosphates.

Are you sure they were diatoms? Diatoms are easily blown off surfaces with a turkey baster. Dinos however generally don't blow away, and many are toxic. *IF* you've mistakenly ID'd your nuisance issue wrong adn they are infact dinos they can definitely impact corals. I had several die when I was fighting them for months.

 

[Idech]

Are you sure they were diatoms? Diatoms are easily blown off surfaces with a turkey baster. Dinos however generally don't blow away, and many are toxic.

I am very sure as I’ve became the expert at Dino chasing on the microscope. I’m used to diatoms as well and that’s what it was. I’ve dosed huge amounts of silicate.

 

[Randy Holmes-Farley Verified Community Expert]

The alk tested higher after my Phosbond was depleted and the phosphates had increased. Does it make corroborated that fact ?

It’s a possible explanation. That’s why I mentioned it. I discuss it here in this article on GFO:
Iron Oxide Hydroxide (GFO) Phosphate Binders by Randy Holmes-Farley - Reefkeeping.com

What else might iron oxide hydroxide do? Precipitation of CaCO3

Many aquarists using GFO have reported unusually extensive precipitation of carbonates on the solid GFO, and elsewhere in the system. Such precipitation can, for example, be a contributing factor in the caking of such materials, and can coat other surfaces in the aquarium. This precipitation can also contribute to a drop in alkalinity and possibly pH as it removes carbonate from the water column. The effect of calcium will be similar, but smaller on a percentage basis, with a drop of only 20 ppm calcium for every 1 meq/L (2.8 dkH) drop in alkalinity. Increased calcification by corals and coralline algae (possibly spurred by reduced phosphate) can also cause similar drops in calcium, alkalinity, and pH.

Dissolution of these precipitates with acid, accompanied by bubbling, indicates that these deposits are carbonates, and are most likely calcium carbonate since it is supersaturated in most reef aquaria (and in the ocean). Several factors may contribute to this precipitation. Many of these are rather straightforward. It is known, for example, that phosphate inhibits the precipitation of calcium carbonate. Much like the role that magnesium plays in seawater, phosphate binds to the growing calcium carbonate crystals, poisoning their surface against further precipitation of calcium carbonate. Many organic materials are also known to inhibit this precipitation. Near the surface of the GFO, and downstream from it, the organics and phosphate are expected to be lower in concentration than upstream from it. The reduction in concentration of these inhibitors may well permit increased abiotic precipitation of calcium carbonate on such surfaces.

Two more esoteric events may, however, be equally important. The first is that the local pH near the GFO surfaces may be higher than in the bulk solution. This effect arises as phosphate and other inorganic and organic ions displace hydroxide from the surface. Figure 2, for example, shows phosphate displacing two hydroxide ions. The net swap of HPO4-- for 2 OH- will raise the local pH. The supersaturation of calcium carbonate increases as the pH rises, driving the precipitation of calcium carbonate.

Another possible role may be played by the iron itself. GFO is not completely insoluble. The solubility of iron hydroxide in natural seawater is small, but still significant (0.02 - 2 ppb), although it is largely controlled by the availability of organic ligands.11-13 One interesting possibility lies in the way that soluble iron actually impacts the precipitation of calcium carbonate.

At high concentrations, iron inhibits the precipitation of calcium carbonate. While different researchers find different threshold concentrations for this inhibition (>25 ppm in one case,14>7ppm in another case15), it is a well established and studied phenomenon. The mechanism is believed to be the same as for magnesium, phosphate, and organics, which all poison the growing calcium carbonate surface.

At much lower concentrations, however, iron actually increases the precipitation of calcium carbonate by acting as a site for nucleation of new crystals. In one case this happened at 100 ppb dissolved iron, increasing the rate of scaling (the precipitation of calcium carbonate on surfaces) by about 60%.14 In another case, the induction time for precipitation (that is, the time it takes for precipitation to begin once the water becomes supersaturated) was reduced by 40% at 1.4 ppm iron and the overall precipitation rate was increased by 32% at 560 ppb (lower iron levels were not tested).15 These studies were carried out in freshwater, and I have not seen similar studies in seawater.

Is the natural dissolution of GFO important in the nucleation of calcium carbonate precipitation? I am not sure. But it is clearly one possible explanation that fits the observations of aquarists as well as known phenomena involving iron.

 

[CHSUB]

1) why is there such a big impact on corals when phosphates rise (they consume less alk and they don’t grow) in my tank ? I see so many beautiful tanks when people say they don’t even test or that their phosphate levels are a lot higher ?

It is well documented that phosphates limit calcification. I personally have experienced this, however that was with > 0.5 ppm po4. I believed stone corals just grew very slowly and at the time 0.5 ppm seemed like a reasonable po4 level. I think at around 0.1 to 0.2 you would barely notice a consumption difference. In most cases when people report “great” growth at x po4, imo that is subjective and not based on any measurement. Beautiful tanks don’t necessarily have great growth, might even have very little growth. My tank with high po4 was beautiful, imo, and I believed growth was typical and good.

2) How do experienced reefers manage this alkalinity/phosphate relation ?

I would replace experienced with smart to answer this question. My answer than would be by keeping po4 low or at least below 0.1 ppm. Small fluctuations are not problematic and new studies show that very low levels, near to or undetectable with hobby testing and short “pulse” additions of po4 is best.