Lowering pH - Strange Problem

[TDWagner]

Fish are adaptable animals, that can be acclimated to a wide range of conditions.
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pH does not matter. dissolved solids levels matter, and to a lesser extent Mineral profiles.
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Dissolved solids are the true test of hardness, pH does not directly correlate to water hardness or solids levels. pH doesn't matter solids do.

Ok, I've got it now.

pH - The number doesn't matter as long as it's STABLE
KH - Buffering capacity...keeps pH stable

Fish are adaptable, but I'm guessing only to a point, right? You wouldn't want to keep hard water fish and soft water fish in the same tank. That's the main factor when choosing what fish to keep in your tap water...HARDNESS!!!

The only point I'm still fuzzy on is how to measure hardness...is it GH or TDS...maybe both? Can I test my GH and then choose hard or soft water fish based on that result, or do I need to get a TDS meter to get an accurate hardness reading?