06-23-2010, 01:12 PM
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Provided you do not overdose plant fertilizers, they will not harm fish, plants or invertebrates in the aquarium. The amount of heavy metals (iron, copper, zinc, manganese, nickel) in fertilizers made for an aquarium is not high enough to cause problems.
Always dose only as recommended on the label, never more whether in amount or frequency. Heavy metals have the ability to build up in the water if the plants do not assimilate them, and while plants also have the ability to detoxify heavy metals beyond those they assimilate as nutrients, there is a limit. I don't know what it might be, and probably it is fairly significant, but why take the risk? Dose as per the label and you'll be fine.
There could be more heavy metals in your tap water that you would add via fertilizer, depending where you live and the source of your tap water. Conditioners that detoxify heavy metals handle this, but they also have limits; the heavy metals in tap water is usually what is referred to as trace amounts, and conditioners handle this; excessive metals are beyond the ability of conditioners, but fortunately our tap water doesn't contain such amounts--the local water authority sees to that.
Chelated means the metal is bound to a dissolved organic substance and is thus more easily assimilated by plants. This allows the trace element (copper, iron, etc) to be slowly made available to the plants over a period of time. Plants require very minimal amounts of trace elements: copper, iron, manganese, nickel, zinc, molybdenum, chlorine and boron to name all of them. Good comprehensive fertilizers have the included trace elements in minimal amounts and in proportion to each other according to the plant's needs. This is one reason I always recommend comprehensive fertilizers rather than dosing individual minerals ad hoc.
Last edited by Byron; 06-23-2010 at 01:18 PM..