@Milliardo Peacecraft You need to get your Nitrates up. IU had this problem when i first started doing planted tanks where my plants would die off because there would be no usable nitrogen for them. Having a nitrate level of 0 would be great for a fish only tank. However, nitrogen is one of the main nutrients for plants just like what @JoeQ said with NPK. (The N stands for Nitrogen.) Nitrates are where plants usually get Nitrogen. I would try to keep your Nitrates between 20 - 40 ppm, but I have had my tanks get above 80 ppm with little to no side effects. Also, you might want to watch your Nitrite levels as they can be harmful for your fish. Remember the most dangerous thing fro your fish is Ammonia, then Nitrites are still bad for your fish but they can tolerate a little more, then Nitrate isn’t harmful until a very extreme level but it is still a good thing to try to keep them under control.