High tech tanks usually require the addition of nutrients to the water. If not to begin with, then eventually. Your dirt might have given out of nutrients.
You could put some of the capsules of osmocote into your substrate.
You can buy dry ferts and mix your own liquids. Can be a little confusing at first, but once you get the hang of it, it's a breeze. I get mine and the dosing bottles from Green Leaf. I didn't look but I suspect the site Guppy mentions above has the dosing bottles also.
You can buy Seachem liquid fertilizers. Great stuff but cost prohibitive for many. However, you have a small tank. Might be something that you can get quick to get you back on your feet, and then switch to something else if you want.
On second thought, my osmocote suggestion is probably not a good one for the short term at least. Your plants are already turning yellow. They need something now. Osmocote is a longer term slower solution.
Last edited by Darkblade48; 09-13-2016 at 03:53 AM.
Reason: Please use the edit function for back to back posts