[Peter Finke]

Mike, I cannot tell you whether your substrate is a good substrate for licorice gouramis since I mistrust any of these commercial products in respect of their composition. But perhaps there is a list of contents given somewhere. Could you tell us? What sort of gravel is the main substance in it? Quartz? Granite? Laterite? Peat? There must be some natural substance (like peat) or some artificial chemical product in it if the substrate is able to reduce the Gh, Kh and pH in such a big dimension that you told us. Until I don’t know the exact composition of that substrate I would not use ist. There may be grwoing substances for plants in it (for what else one needs such a substrate?) that may be of use in a standard tank with standard fish but of evil in a tank for blackwater fish.
The reduction of Gh from 11 to 6 is certainly good for our fish, or better: Gh 11 is absolutely of no use for them. But the more important values are the two others. The Gh is of no great value since it could consist of several different components. Licorice gouramis are not accustomed to any calcium in their waters. Therefore a Kh exceeding 1 is entirely unbiological. In this respect the reduction you told us is highly recommended. The same holds for the reduction of pH from 7.8 to 6.4. Any alcalic value is to be avoided because of it’s inevitably instigating the germs to propagate; the lower it is the lower is the germ-content. Eggs of our fish will not develop with a Kh more than 1 and a pH above 6.8 or even 7.
But I have another question to you. How do you measure that water values? The background of my question is that I mistrust that huge reductive powers of a commercial product which we do not know the composition of. Reducing the pH in presence of calcium is chemically highly improbable. Well, the calcium has been reduced too, that might explain that result. But by what substances? The question whether that will be a lasting effect is not to be answered without knowledge of the components. It is highly probable that it will not last. But by what method do you measure your values? Many methods will produce mistakes, even electronic equipment. If not calibrated very exactly, you receive fantasy numbers. Therefore it is good to know: The exact hardness and the exact pH is less important than the certainty of of pH below 6.8 (and above 3.0, but it is nearly impossible for you to reach that mark) and the certainty of having reduced calcium to nearly nil.
My last questions: Why do you need a substrate at all? For growing plants? That is difficult in a water of high acidity and a hardness near to nil. And adding plant nutrients will change your water values again in a direction not useful for the licorice. Of course there are plants in the natural habitats of our fish, but mind: That are flowing waters, and the plants are rarely true underwater plants. The majority are plants that build their leaves above the water surface (grasses for instance). They don’t affect the chemistry of the water to a markable degree, they are simply “structure”. There are only a few true underwater plants, but the flowing waters will carry the new nutrients and will carry them away, too.
But anyway: I have a substrate in my small licorice-tanks too. But it consists either of pure (!) peat, or of a very thin (max. 1 cm) layer of quartz or fire-clay gravel, and old beech’s and oak leaves. I take it to be a settling-structure for useful bacteria. They settle on all surfaces, but a thin layer of a gravel enlarges the surfaces considerably. I have plants in my small tanks too, but only javamoss and other moss, a few shoots or branches of Ludwigia or Rotala rotundifolia, rare a small flowerpot with a Cryptocoryne, and of course floating plants. I always recommend Ceratopteris, a plant that is highly valuable in a tank that need constant germ-reduction. There are more plants to be ued of course, but’s a science of its own.