For me, with tap water with a KH of 1.5º, the increase in conductivity is almost
negligible. For people with really hard water, the pH can only be lowered by
removing carbonate hardness. This will require more acid than in soft water, and
the increase in conductivity is not a good idea. HCl doesn't increase the TDS
very much because chloride ions are not a strong hardening agent. Chloride ions,
however, will raise the electrical conductivity considerably. I wouldn't suggest
using HCl to lower pH without checking to see what it does to your conductivity.
For most soft water fish, increased conductivity is just as detrimental as high