Aquatic plant fertilizers are formulated differently than terrestrial plant fertilizers. Plants in the aquarium often have ammonium, nitrate, and phosphate in abundance from fish waste, so typically you do not want much of these elements in the aquarium fertilizer, otherwise you will likely have a very heavy algae bloom. Plants require major and minor trace elements to grow and reproduce. Trace elements and/or light are the limiting factor in plant health. Plant growth can be stunted when any one element that it needs is in short supply. Different species of plants require different levels of trace elements to thrive. In some cases, some plants in the aquarium will seem to do just fine, while others will struggle. In some cases, just the addition of a liquid iron into the aquarium system will make all the difference in plant growth.
A good all purpose aquatic plant fertilizer will have higher values (major trace elements) of iron (Fe), potash (K2O), magnesium (Mg), sulfur (S), sodium (Na) and calcium (Ca). There are many minor trace elements, of which nitrogen and phosphate should be only in very minor amounts.
Liquid plant fertilizers are normally added to the water column. My preference is to inject it into the sand bed with a plastic syringe. The fertilizer is mixed with about 20 parts aquarium water, drawn into the syringe, then a small amount is injected every two inches (5 cm) into the bed. This helps keep most of the nutrients in the bed, and away from algae.
Injecting the sand bed with aquatic plant fertilizer. This technique delivers the fertilizer to roots, and away from algae.