The lower limits of Mg


Jan 24, 2005
Cologne, Germany
Hi Tom,

your rec. for Mg supply reads 1/4 teaspoon ~ 1,67g MgSO4/week/20g. This rounds up to 22ppm MgSO4 or 2,2ppm of pure Mg.

From what I've observed I say this: With hornworth in my tank (a real fast grower) introducing 3,5ppm of pure Mg/week did not support growth of Rot. rotundifolia in the long run. Other data: KH 2, NO3 5-10ppm, PO4 0,4ppm, lighting 2,7 watts/g, press. CO2 (mist).

After months of introducing 3,5ppm Mg/week (other param's as stable as possible) I measured only 2,2ppm Mg (GH 2,3, Ca 12,4ppm). Supposedly hornworth has been taking up a considerable amount of Mg. Rot. rotundifolia resumed growth from only 1cm/week to normal rates (>5cm/week) after I added one extra ppm of Mg (MgSO4 and MgCl) during a 3 day period and keeping Mg levels there (4,25ppm) after wc's. Sounds weird to me as only 1ppm Mg makes up for such a difference. From what the plants are telling me I'd rec. keeping Mg round about 4ppm min. (might be related to KH etc.). From my observations I derive also that there is a distinct difference in Mg demand. Hornworth does not need as much Mg as R. rotundifolia.

Please take these findings with great caution since I have NOT repeated the test nor can I exclude measurement errors. Just my 2 cents since we don't know much about min/max amounts of Mg.


Tom Barr

Staff member
Jan 23, 2005
Re: The lower limits of Mg

I think based on some of the references, you can easily dose more, but you should see a significant improvement with the 1/4 per week. 2.2 ppm a week is not a lot of Mg, but then again, other products also add Mg. I use an all in one product for GH, it adds SO4, Ca and Mg (more than SeaChem).

If you found adding another 1-2ppm works, your tap is really soft from what I understand, then there should be no issue.

SFBAAPS found adding about 2-4 degrees of SeaChem eQ solved any Ca,SO4, Mg issue.

I add about 2 degrees worth.

You can individualize the GH components to get a better handle on it, the reason Mg is such a popular issue, is that folks tend to have Ca as the lion's share of their GH in the tap water.

In designing a trace, macro, GH etc, you want to add a little K and Mg and Ca in most of these.

These do no harm at the higher levels near `as I can tell for Ca and K, I have not done extensive Mg work personally.

I have plans to torture Rotala wallichii and R "green" indica with Mg and soft water. We have not had issues with Rwallichii at 3-4 GH from SeaChem EQ ever. The plants grows very rapidly.

So higher levels seem to do well with all the K, Ca, SO4, but perhaps individually Mg plays a different role if it is high relative to the other GH/salts.

I pick sensitive plants to do these test on for this reason, then you can assume the rest of the weeds will do fine over a wider, broader range.

Tom Barr