Perhaps I've had the wrong idea of what my fish need all along. Some searches come up with these general ranges for my main tank inhabitants;
Buenos Aires Tetra;
Hardness: to 35 dGH
pH : 6 - 7
Water Hardness : 6° to 15° dH (I assume that's dGH?)
if so, my GH at 7 could go higher, as well as my KH - but keeping the pH in the Colombian Tetra's range might require a bit of peat, oak leaves, etc. if it goes too high. I already have massive amounts of driftwood in there, so maybe that will buffer sufficiently.