Magnesium can be toxic at concentrations approaching natural background levels, but toxicity is dependent on Ca concentrations. You could calculate the magnesium:calcium mass ratio. I would try to hypothesise that a ratio of 9:1 could be suitable for the species that you are trying to grow. If you did not achieve these levels within you existing magnesium concentration then I could see some justification in adding calcium. You would not need to add any more magnesium because it would exceed the concentration of this trace mineral that is typically found in dried plant matter, and it is fully mobile in solution. Others research suggests that oligotrophic conditions, and low levels of magnesium, have no bearing on plant growth. It just needs to be present in very small amounts and the plant will decide how much it will use.
With relation to your potassium (kalium) and nitrate measurements, they are within the ranges recommended by estimative index dosing. This is some reassurance. As expected, your phosphate level is just below the recommended range. However, this too is a mobile mineral and the plant can happily use, and even store, as much of it as is required. Hypothetically, the single biggest factor affecting your nutrient deficiencies, you hinted to above when you mentioned PNK ratios. These ratios were actually developed by the Rothamsted Experimental Station for terrestrial crops. In those systems, potassium does play a big factor because it is easily leached from soils. It is less relevant to out hobby because potassium can easily be held in suspension. For aquatic systems, instead, we find that carbon is the biggest factor of plant nutrition. Wetzel gives overall values of 1000 C : 175 N : 25 P for nutrient cycling in river ecosystems. So if we were to take typical values and apply that to the ratio of nutrients in your tank, we would find that the ratio should be 1 P : 7 N : 40 C. Now its not fair to say that this must be the ratio between phosphate, nitrate and carbon dioxide - because there are many chemical species involved. But if you had low levels of carbon dioxide, then this limitation would probably contribute towards far lower rates of nutrient uptake (mineralisation) and slower growth. Apart from falling within estimate index ranges, this is further evidence to support that you have adequate nutrient ranges for your particular set up (notably phosphate). Deficiency is due to plant health and not water chemistry.
Nobody really bothers with studying how specific plants respond under specific conditions to specific level of nutrients because this is almost impossible to replicate. Even phenotypes of the same plants can behave radically different. All we can say is that you have broadly the right chemical conditions. If we must delve into assumptions on what nutritional parameters work, then the best evidence is anecdotal - that is - if you follow a dosing regime, then you have nutrients in abundance. You follow a dosing regime, and your nutrients are present. Go ahead and gradually increase these levels to see if this triggers the plant into recovering. If the chelates inside plant vascular tissue were missing or malfunctioning, then higher concentrations could help - it is unlikely you will do very much harm. Personally I would start with a little bit more phosphate. It is my belief however that immobile nutrients were more likely effected, because if your roots were no longer in a preferable environment, then the transportation of these nutrients could have shut down. Try increasing you oxygenation - this often benefits root conditions.
