To support optimum plant growth, aquarists sometimes add fertilizer at initial tank set up. This will promote strong root system, the absorption of nutrients and overall health of the plant. Sometimes, root tabs are also added periodically after initial set up to maintain the look of the plants, particularly the strong root feeders, like Amazon swords. There are commercially available root tabs but they are not cheap. Seachem Flourish Tab costs $20 for 40 tabs. The cheaper option is using the slow released fertilizer available in garden center. They are encapsulated in granules but they are easily breakable and released the fertilizer into the water column while inserting them into the substrate. The trick is encapsulate them into gel capsules and insert the gel capsules into the substrate. The gel capsules will be melted after immersed in water thus exposed the slow released fertilizer.
I used Osmocote plus and size 00 gel capsules. The cost is about the same but I can make thousands of them.
Signs of plant nutrient deficiency: