Previously I've mentioned about the belief that kalk reduces phosphate although I've come across no rock hard evidence beyond the theoretical. I certainly noticed rock formations in my little sand bed in the sump directly below the dosing line a while back, before I got rid of it.
I thought it was worth trying something again. I was dosing 1300 mls of kalk fortified with 90 mls of vinegar per gallon, into the aerated portion of my sump. I moved the dosing line to within 2 inches of my bag of sand (without regenerating it). Gently mixed with tank water this flows over the sand bag.
I think I'm growing rocks.
Tank pH seemed not to get quite as high as normal, and alkalinity consumption jumped a little to requiring 100 mls of vinegar in kalk along with 0.1 to 0.2 DKH 2 part daily. The interesting part is that phosphate has decreased from 0.33ppm to 0.23 in 3 days. Normally an overdue sand bag change would see a slow rise at this stage.
Thought it was worth noting anyway
Before changing the position of the dosing line I did a test with tank water, adding 1% of kalk (90mls of vinegar / gallon) and checking the phos on that. Went from 0.33 to 0.26, for what it's worth.
Yes, it has long been touted as a possible benefit, and Craig Bingman showed it in limewater itself, but not in tank water it was added to. A measure in tank water would be interesting to note, and you're growing rocks may certainly be a factor.
This is from one of my limewater articles:
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.
