Good discussion, glad to see the advanced forum getting some use.
A few comments:
>Problem with all mag suppliments out there. (no matter which one you use or prefer due to cost or whatever) They have high levels of phosphates....<
I was not aware of this. It would be worthwhile to know how you did your testing. Testing a pure solution of concentrated magnesium chloride with a standard test kit (even the Hanna meter) might give you a false reading due to the test kit not being designed for testing a high concentration solution (not saying you did this, I'd just be very careful with testing like this).
A good way to test for something like this (I think) would be to add enough of the magnesium supplement to an aliquot of water from your tank to increase the magnesium by 200 ppm or so. Then see what the Hanna meter gives you for a change in PO4 levels. I'd say if it was less than a 0.02 ppm increase, it would not be a big deal. The Hanna meter should be able to test for this level of increase. In the initial testing of the Liam's meter that I did before the club purchased the meter it
was sufficiently sensitive in this range.
>The other problem....
If you try to add too much & your water is already quite saturated you will end up with percipitation...(the sandbed will then be saturated with the suppliment & will add levels of phospahtes to toxic concentrated levels) <
Actually, if you are talking about magnesium I don't think that precipitation is likely to occur in the ranges that we are talking about (a 100-400 ppm increase in magnesium). I bumped magnesium up to around 1500 once with no apparent issues in an isolated tank. Magnesium carbonate (unlike calcium) is not supersaturated in seawater. If in fact a lot of phosphate comes in at the same time, I'd agree that the phosphate would be a big problem just to the coral health. But phosphate actually 'poisons' the growing crystal (stopping it's growth) when calcium carbonate is attempting to precipitate in seawater. This is why high phosphate is bad for corals.
In talking about calcium preciptation events linked to an addition of a seed crystal for precipitation to start on Randy has this to say about the interaction of magnesium and calcium (from
http://www.advancedaquarist.com/issues/mar2002/chem.htm):
What processes inhibit continued precipitation of CaCO3 onto a growing crystal? The main thing happening in normal seawater is likely the impact of magnesium. It does two critical things:
1. Magnesium holds onto carbonate ions and reduces their free concentration, thereby reducing the likelihood of precipitation onto calcium carbonate surfaces.
2. Magnesium gets onto the growing surface of the crystal, essentially poisoning it for further precipitation of calcium carbonate.
Note that while both of these processes inhibit precipitation of calcium carbonate, the first actually increases the solubility, whereas the second does not. It is worth noting that the solubility of calcium carbonate in seawater is about 26 times higher than in freshwater at the same temperature, and this first effect of magnesium is one of the reasons.
>It can also harden the sandbed causing a biological bacteria die off from suffocation. (the top level of the sand will harden from the excess suppliment)
BTW...this can happen with Ca suppliments as well!!!!<
I think that it would only happen with calcium supplements. And only if alk and calcium are high, and also if pH is high at the same time. This is usually only a problem with newly set up tanks with a lot of new, clean fresh sand. Once the sand gets a bit of a natural layer of proteinaceous 'stuff' on it's surface, calcium carbonate is much less likely to precipitate out on it.
>You can't get the phosphate reading from a kit that these mag sups contain....They are loaded with inorganic phosphates...That form of phosphate isn't detectable to the reef test kits on the market....<
While I believe that the form of phosphate that is likely to be in these supplements is in fact inorganic, I believe that it is in fact
organic phosphate that cannot be easily tested for.
Some more from an older article by Randy (
http://www.aquariumfish.com/aquariumfish/detail.aspx?aid=2276):
A second point about organic phosphates is that they will mostly not be impacted by phosphate-binding materials sold to the aquarium hobby. Consequently, while these may do a fine job of reducing inorganic phosphate, they won’t help an algae problem that is caused by organic phosphates.
A final point is that organic phosphates will not be detected by any hobby kit unless it is specifically designed to detect them (most do not). Those that do (e.g., Hach PO-24) break the phosphate off of the organic and thereby convert it into inorganic phosphate prior to testing.
FWIW, I went back and looked at the elemental analysis of the Dead Sea Works magnesium chloride that we purchased several years back (most sources of magnesium chloride would be similar, don't know how magnesium sulfate would compare, but it's best to use a mix of the two, fwiw):
Element Typical(ppm) Maximum(ppm)
Mg 115000(11.5%) 125000 (12.5%)
Ca 7000 8000
Br 5000 8000
K 2500 3500
S(SO42-) 180 200
Sr 120 150
B 20 30
Li 10 15
N* 4 5
Si 9 15
Fe 3 10
P 2 3
I believe this analysis was performed by ICP. It lists P (phosphorus) as a maximum of 3 ppm (typical of 2 ppm), compared with 115,000 ppm for magnesium. An ICP analysis would burn up and convert any phosphate molecules to phosphorus atoms I believe. Using this analysis with the 'typical' value for P, an addition of sufficient MgCl2.6H20 to increase the Mg level in your tank by 200 ppm would increase your phosphorus levels in your tank by 0.0035 ppm, or 0.0107 ppm phosphate. So, looking back at all this now, if you dosed enough MgCl2.6h20 to increase the Mg by about 400 ppm, you'd probably Just barely be able to detect the difference using the Hanna meter (0.02 ppm increase).
I bet if you took a concentrated amount and tested it the same way that you would get a phosphate reading. 600-800 ppm in something that concentrated is nothing. When you add that to your tank you are probably bumping the phosphates up in the parts per billion.
