Chloride vs. Chlorine

> From: "Williams, Rochelle - DCSPIM" <williaro at ftmcphsn-emh1_army.mil>

> Mark Fisher posted on 9 Jan 97, that chloride in general is harmless to 
> fish.  The specific case mentioned was the addition of table salt, Sodium 
> Chloride, NaCl.  I've a Phosphate garden fertilizer that is made from 
> Phosphate Chloride, KCl, but I thought any form of Chlorine (i.e. chloride) 
> was bad for fish.  Does this mean the Phosphate fertilizer is safe to use 
> in an aquarium substrate?


There are a few issues here.  First, I'm not sure what is in the 
fertilizer that you have.  You identify it as a phosphorus-contaning 
product, but you mention the ingredient potassium chloride (KCl.)

Phosphorus and potash are very different things.  phosphorus is found as
phosphate in biology, it is an ingredient in DNA, RNA, nucleotides,
phospholipids and enzymatic cofactors.  Potash is another word for K2O,
potassium oxide.  Potassium is a cation (+charged ion), similar in
chemical properties to sodium.  So KCl is like table salt, NaCl, with
potassium substituted for sodium. 

I guess the other major issue is chloride vs. chlorine.  Chlorine is the 
element, it is a strong oxidzing agent, when dissolved in water at 
around neutral pH, it is largely in the form of hypochlorite ions, which 
you may recognize as the active ingredient in bleach.  Chloride ions are 
essentially unreactive.  It is the major anion in seawater, but usually 
it isn't the dominant anion in freshwater, or it is about as abundant as 
sulfate or bicarbonate.

Some fish from very soft water may be somewhat susceptible to high Cl-.  
Generally, though, at low concentrations, it is completely innocuous.  I 
doubt that there will be enough Cl- buildup from the use of the 
fertilizer that you mention to cause problematically high Cl- 
concentrations if you perform even occasional water changes.