Thanks for your reply. I know, that it's possible to add teaspoons of KNO3 and KH2PO4 weekly to my tank and growth will be fantastic. On the other hand I don't want to have concentration of NO3 up to 50 ppm. Why? Maybe it's good for plants, but there are some fish in the tank too. I know, that high NO3 concentration can't cause problems immediately, but in a long term it's not good at all for livestock. All discus breeders try to cut nitrate levels as much as possible, because growth and healt of fish is far better. That's why I use NO3 test kits. I currently try to keep my nitrate levels around 30 ppm. Hobby test kits are more accurate than you think - unfortunatelly quite expensive (I tested JBL NO3 test and JBL PO4 test). I tested these tests, I used KNO3 to calibrate them and they were +- 10 ppm accurate. I don't care if there is 20 ppm or 30 ppm, it doesn't matter. Without these tests, I wouldn't be able to find out approximate nitrate level in my tank.I can guarantee you that using test kits cannot help you to grow plants.