This article is from the Aquaria: Plants FAQ, by multiple authors.
If you like keeping zillions of test kits, then you can check some
trace element levels with them (Dupla recommends an iron level of
0.1ppm). Ammonia and nitrate test kits will tell you if you are
overfeeding. Alternatively, you need to watch your tank. Too much
fertilizer and fish food may show up as excessive algae growth.
 
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