Giving Kalkwasser a Try.

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.

DonnieP wrote:I agree, a lot of work for some colored sticks If you do the two part I would suggest C-Balance, it contains all the trace elements as well and has been proven with reef crystals by the fella that grows fantastic sps reefs in a jar, you may know him, names Brandon, he convinced me a long time ago that C-Balance and reef crystals were a perfect combination. Either way, its gonna get expensive with the two part, I been going that route thats why I am trying Kalkwasser. I have tried the Kent two part but it seems like alk and calcium drop right back out the next day. I have been using C-Balance exclusively on the pico and the params stay where they should and PH never drops below 8.1 day or night and alk is always 8-9 calcium 420. I'm gonna keep up the kalkwasser for a month or so and see how it goes, everything else looks fine. Gold maxima has bunches of new shell, blue is kinda staying the same but I do see new growth.

dahenley wrote:whats your PH and ALK?
Kalk raises your PH.
but it also adds Calcium and ALK to your water.
If you have lots of Evap, you can over dose ALK to your tank.

ritter678 wrote:
It ran overnight and so far no problems, ph 8.0 (which in the morning is higher than I've tested in my tank, usually 7.8 in the a.m.), alkalinity was 8.23 dkh.

ritter678 wrote:I checked the alk this morning and it was actually slightly lower, 8.02 dkh.

ritter678 wrote: Ph is 7.8 in the morning and over 8 in the evening, which is about where it was before. My alkalinity is staying the same with the kalk as it was with the baking soda.

ritter678 wrote:Calcium is at 400 this morning and I added another dose so I should be getting back up to target range in a day or two. Alkalinity was at 7.34 this morning. I didn't check the ph. I upped the kalk dosage in the new jug to 3 tsp.

ritter678 wrote:Finally got my calcium up to 460. My alk was dropping when I was using 2 tsp/gal. kalk but it's been locked in for the past few days since raising the solution to 3 tsp/gal. For about the past 5 days it has only tested between 7.22-7.34. I may bump it up to 8-9 with baking soda so I have more room for error. I'm going to wait a few more days and see if I need to adjust the kalk dosage though.

ritter678 wrote:I bumped up my alk to 8 with baking soda and my numbers have been solid since upping the kalk to 3 tsp/5 gal. For the past 2 weeks my alk has been 8-8.35, cal. 460.

ritter678 wrote: My parameters as of yesterday are; Cal. 460, alk, 7.84, ph, 8.0 morning and 8.2 evening. I even tested magnesium, I didn't get an exact number (my test only goes up to 1250) but the levels were within range. Salinity 1.025, no ammonia, nitrites, nitrates, phosphates detected.