This article is from the Aquaria: Plants FAQ, by multiple authors.
Fish food usually provides enough of the three macronutrients,
nitrogen, phosphate, and potassium (N-P-K), to keep your plants
healthy. However, the trace elements such as iron are not all supplied
in a form that the plants can use. Some trace elements may be in your
tap water, so frequent water changes will replenish them. This may
provide enough for some plant growth, but if you want the best growth
you should consider adding a trace element fertilizer.
 
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