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RE: [APD] pH shock

Secondary affects of pH are a separate matter from the pH change itself, as are changes sufficient to damage the fish directly.  Examples of these are ammonia<->ammonium, or extreme pH changes, eg a ph of 1 is likely to cause fishes significant distress, but that is regardless of how slowly or quickly it approaches 1.  Similarly, the chemical that causes the pH change may itself be a problem, but the pH change itself not be a problem.  For example, adding uranium carbonate in sufficient amounts will kill the fish due to heavy metal poisoning and/or radioactivity, but not likely due to the pH shift.  This whole area of multiple causes and secondary affects is what makes understanding these issues so difficult, and is often a reason that these myths start.  
Several people have testing ph swings from ph 4.5 to 8.5 within minutes, in CLEAN water, ie no ammonia, and back again, and the fishes do not appear to notice the affect.  That strongly suggests that the problems are secondary, not primary.  Still, this was with specific fishes, and there might be other fishes (eg perhaps those that generate strong electrical fields) that maybe much more strongly affected.  There just isn't enough information yet to know about the affects.

Similarly, fish like discus that "require" low pH levels, may in fact not require a low pH level, but only require low concentrations of disolved compounds (something that is known to strongly affect cells directly, via osmotic pressure), but I don't know of any actual studies that attempt to figure out what the actual requirements are.  (On the other hand, it is entirely possible that it is actually low pH levels, and not low disolved compounds that is needed - if the problem is that the low pH water participates in some chemical equilibrium reaction internal to discus.)


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