[Peter Finke]

Excellent reply by Martin. I cannot tell it better.
But I can affirm everything, especially his observations on the relative pH-stabiliy. Just contrary to the general opinion the low pH is rather stable in my small Parosphromenus-tanks, too. But one should observe three things: very low conductivity, presence of organic materials that add humine substances, and restriction to plants which make use of nutrients that are solved in the water without having a great chemical activity (that is, for instance, floating Ceratopteris. A tank which is planted heavily in the conventional way may contain too many plants that influence the pH. Ceratopteris or Javamoss seems to be ideally suited for our purposes. A few others will not damage the system).