Can you use Kalkwasser to lower PO4?

[CayoHueso]

Is Kalkwasser a tool available to use to lower phosphate levels? Further, do all 2 part Alk & Calc additives bind with, thus lowering, phosphates?

After watching a Tidal Gardens video that mentioned using Kalkwasser as a way to lower phosphates, I came across some information and some older posts on Reef Central from @Randy Holmes-Farley regarding Kalkwasser's ability to bind with PO4 at higher pH levels and precipitate out as calcium phosphate.

I ask because currently I have low alkalinity levels in my tank (6.2 dKH) and high(er) phosphates (0.24ppm). My Calc is normal (422ppm). If dosing Kalkwasser has the potential to raise my Alk levels while simultaneously lowering, or keeping in check, my PO4 levels that would be great choice for my low Alkalinity issue.

Do all two part Alk & Calc solutions also bind the Calc & PO4? OR are the reductions in PO4 negligible at the pH we'd like to keep our tanks at?

 

[Randy Holmes-Farley]

While there may be some effect, it is pretty minor in most cases and I’d look to other methods instead of or in addition to it if you want a significant reduction.

 

[CayoHueso]

While there may be some effect, it is pretty minor in most cases and I’d look to other methods instead of or in addition to it if you want a significant reduction.

Is it just Kalkwasser? I was leaning towards Reef Code A & B, because my Calc seems to be good (I'm leaning towards Reef Code B alone, with just one dosing pump). And I'd like to avoid adding the Kalkwasser to my ATO, so I would need a dosing pump and container anyway. I have an AIO, so a Kalk Reactor would be difficult. But if Kalkwasser has this added benefit, even if a little, I may choose Kalkwasser over two part.

 

[Randy Holmes-Farley]

Is it just Kalkwasser? I was leaning towards Reef Code A & B, because my Calc seems to be good (I'm leaning towards Reef Code B alone, with just one dosing pump). And I'd like to avoid adding the Kalkwasser to my ATO, so I would need a dosing pump and container anyway. I have an AIO, so a Kalk Reactor would be difficult. But if Kalkwasser has this added benefit, even if a little, I may choose Kalkwasser over two part.

There are two hypothesis of how there "might" be an effect. There is no data in a reef tank that I am aware of. Both, however, depend on the high pH of kalkwasser (or hydroxide two part, which might be even larger due to the higher concentration) driving precipitation. A carbonate two part may have some similar but smaller effect. A bicarbonate two part is unlikely to have much effect.

https://reefkeeping.com/issues/2005-01/rhf/index.htm#15

What Else Does Limewater Do In An Aquarium? Reduce Phosphate

Many reefkeepers accept the concept that adding limewater reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and has published them in the old Aquarium Frontiers. While many aquarists may not care what the mechanism is, knowing it would help to understand the limits of this method, and how it might best be employed.

Habib Sekha (Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef aquaria. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef aquaria will exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many find that calcium and alkalinity levels are stable over long time periods with just that scenario. One way that can be true is if the excess calcium and alkalinity that such additions typically dump into the aquarium are subsequently removed by precipitation of calcium carbonate (such as on heaters, pumps, sand, live rock, etc.).

It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels: phosphate binds to these growing surfaces, and becomes part of the solid precipitate. The absorption of phosphate from seawater onto aragonite is pH dependent, with the binding maximized at around pH 8.4 and with less binding at lower and higher pH values. If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can inhibit the complete removal of excess phosphate from a reef aquarium that has experienced very high phosphate levels, and may permit algae to continue to thrive despite having cut off all external phosphate sources. In such extreme cases, removal of the substrate may even be required.

If the calcium carbonate deposits are growing, then phosphate may get buried in the growing crystal, which can act as a sink for phosphate, at least until that CaCO3 somehow dissolves. Additionally, if these crystals are in the water column (e.g., if they form at the local area where limewater hits the tank water), then they may become coated with organics and be skimmed out of the aquarium.

An alternative mechanism for phosphate reduction via limewater may simply be the precipitation of calcium phosphate, Ca3(PO4)2. The water in many reef aquaria will be supersaturated with this material, as the equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. The supersaturation of calcium phosphate will be even higher in the high pH/high calcium fluid present where limewater enters reef aquaria. The locally high pH converts much of the HPO4-- to PO4---, and it is the concentration of PO4--- that ultimately determines supersaturation. That high supersaturation may tip the balance to precipitation of calcium phosphate, just as too much limewater all at once can tip the balance to precipitation of calcium carbonate. As with CaCO3, the precipitation of Ca3(PO4)2 in seawater may be limited more by kinetic factors than by equilibrium factors, so it is impossible to say how much might precipitate under reef tank conditions (without, of course, somehow determining it experimentally).

As with the precipitation of CaCO3 containing some phosphate, if these calcium phosphate crystals are in the water column (e.g., if they form at the local area where limewater hits the tank water), then they may become coated with organics and be skimmed out of the aquarium.

 

[rhitee93]

There are too many variable for this to mean much, but I have started dosing kalk in the last few weeks, and have been slowly turning up the rate to try to match my consumption. I'm up to about 2.5L of saturated limewater a day at this point.

Over the last 3 or 4 weeks I have also had rising phosphate levels and have had to return to dosing lanthanum chloride to get it down. (I had this issue a few months ago, but had stopped the LC for quite some time)

I'm not saying kalk has anything to do with phosphate going up, nor can I say the kalk isn't helping. However, they "feel" pretty independant to me at the moment.

 

[Garf]

There are too many variable for this to mean much, but I have started dosing kalk in the last few weeks, and have been slowly turning up the rate to try to match my consumption. I'm up to about 2.5L of saturated limewater a day at this point.

Over the last 3 or 4 weeks I have also had rising phosphate levels and have had to return to dosing lanthanum chloride to get it down. (I had this issue a few months ago, but had stopped the LC for quite some time)

I'm not saying kalk has anything to do with phosphate going up, nor can I say the kalk isn't helping. However, they "feel" pretty independant to me at the moment.

I'll add anecdote that I've actually been reducing kalk additions (and tank pH) and noticed no increase in phosphate. I don't keep a record of what I add, or weigh the amount of food I add to the tank however.