I sent our discussion on Magnesium to Claus Christensen for his 
comments.  Here is his response:

Magnesium in Aquarium

First of all Magnesium is a very important fertiliser for all 
Also for Aquarium plants!

The point is that many places in the world they get to much or to 
little of Mg from the tap water!

If you are living in an area with to little Mg it certainly will 
help the plant if you add it!
Opposite if you are living in an area with too much Mg it change 
for the worse!
So the results depend of what you already have!

Plants can grow in a wide range but optimum condition for many 
plants is between 5 and 25 mg/l. 
NB It is easy to find plants living outside this interval.

Magnesium (Mg) can block the uptake of some other nutrient! 
Specially it blocks for Potassium (K) which is VERY important 
nutrient for water plants!
I think it is one of the reasons when additions are negative!
Magnesium (Mg) corresponds also with Calcium (Ca).
Magnesium is not very easy to measure for an aquarium owner. 
In Europe we can get an analyse from the local authority.

Karen Randall
Aquatic Gardeners Assoc.
Boston, MA