If our water weren't softened, I'd probably not mess with it.... but I sort of figure that water we can't drink and that kills yeast is probably not good for fish.
It's scary scary water -- if you leave a cup of it out on the counter to evaporate you get salt crystals in the bottom.
On the other hand, as my land lord pointed out when I asked about it, it is less expensive to pay a few bucks more rent a month for the water softener then it would be to move every other year because the pipes need to be replaced because of
calcium deposits.
So..... six of one, half a dozen of the other, and I I figure I can afford another buck a month for additional RO water for the tank, on top of what we buy to drink and cook with.
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I don't use RO water though--anyway, the fish at the lfs live in the same water as mine. Personally, I don't think pH is that important unless you're keeping very delicate fish or breeding, say, tetras. Hey, I've kept 2 Otos without changing the water at all.