Ph

Knowing what your pH is now and what fish you want to keep will really determine if you need to modify your water. Most fish can be acclimated to a lower pH just fine. Modifying water isn't the easiest thing, and you have to be consistant or you stress your fish. Increasing pH is usually easier, because you can add things more easily than you can remove them, BUT you still need to determine if the effort is justified.
 
Make sure you understand what's coming out of the tap, and that your Ph isn't low due to other factors. High Nitrates will push the Ph down, so make sure you're not just treating the symptom.
 
OG - Because the nitrification process eats KH. It is not an effect of the nitrate titer, but a result of the conversion process of ammonia to nitrate, which is acid-generating. Two milli-equivalents of bicarbonate are used for each milli-equivalent of ammonia oxidized to nitrate.

With "normal" tank manitenance, enough water should be exchanged in an interval such that the pH decline is not a factor and the nitrate titer stays at a low level. Note that I put "normal" in quotes... ;)
 
Pshew! Thanks RTR for the technical explanation, because I sure couldn't have explained it in those terms. ;) It's just something I'd been told, and that I've witnessed myself, particularly in unplanted aquaria.
 
Okay--knew that the biological process the results in nitrates will lower pH, but the nitrates themselves are not the cause.
 
OK then, we're back on the same page -

I probably should have added that a high nitrate titer is itself a good hint that not enough water changes are being done, so the depletion of the water's alkinity is not being refreshed either.
 
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