By mixing nitrates in a seperate bottle it is much easier to custom tailor your dosing to your tanks need. Often after you dose for some time and keep track of your weekly nitrate and phosphate levels you will find you might need more or less of say nitrates so by keeping them seperate from the phosphate/potassium you can adjust how much you need. For example my tank sucks up phosphate/potassium due to the high number of non-green plants I have so I have to double dose the normal level of phosphate/potassium macro mix daily. I also run a very low nitrate tank and only want 10ppm of nitrates so I only usually dose nitrate a couple times a week. When all your macros are mixed together you can't make any specific adjustments. I'm actually the one who's been encouraging people to split their macro's up this way. Now you don't have to do that but it can be very helpful in the long run. I buy this package
Micro & Macro | Aquarium Fertilizer | Green Leaf Aquariums. Then ordered 3- 1000ml dosing bottles. Also you'll see that the macro mix has MgSO4 magnesium in it but unless you have very soft water with a GH under 4 you don't need to add magnesium into your macro mix. Most tap water has plenty of magnesium in it.