Mag and Phosphate


Premium member
Hello all, hope everyone is safe. Dropped by BLC today, had to pickup salt. The tank is settling down now. Gravel stays clean, every few days clean the glass. Did lose one SPS, not sure why, everything else is doing fine. So here’s the question about levels, all Salifert;

PH - 8.0
Nitrates - .02
Ca - 490
KH - 8.9

Now here’s the rub,

Mg - always off the charts, 1500 plus (test now expired by date on box)
PO4 - always zero, I have seen it register ever. (test close to expire by date on box)

Are the Mg and PO4 numbers even possible? Do they even make sense. I know for the Phosphate Hanna would be the way to go, just was hoping to keep it as a “test”....


Well-known member
Actually yes, this is more common issue than one might actually think

Mag is used by corals when they grow, depending on the salt and if you are doing regular water changes mag could be at 1600 or so and if the nitrates and phosphates are not in sync corals wont grow hence the high magnesium and untraceable phos

Feed a little more or increase your fish load a little., continue testing phos if its an sps system.

Do what ever you do very slowly with out shocking corals.

Even sps when acclimated to slowly love nitrates in the range of 2ppm and should be ok with phos levels around 0.1. Dont worry too much about mag unless its very low

Just my 0.0000002 cents

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