[MarcinB]

My tap water contains about 100pm of Ca and 8-9ppm of Mg (analysis was made in a professional lab) so the Ca:Mg ratio is very high, about 12:1. IMO such a high Ca:Mg ratio can induce Mg deficiency in some plants. I used to change water to a mix of a tap and distilled water in a ratio of 3:1. So the concentration of Mg should be around 6ppm. I also dosed 1,5ppm of Mg weekly from my trace mix, so the water should contain enough Mg for plants. At that time I had some problems with Ludwigia repens and cabomba. Addition of MgSO4 solved those problems quick and completely.