Interesting question. I don't have a water softener, so I'm not sure. Have you tested your water that is straight out of the tap and not treated? Even if it's hard, that might work slightly to your advantage with a sw
tank, whereas it wouldn't really in a fw
tank. The only thing you may have to be worried about is if the water contains other things that are undesireable, such as nitrates, phosphates, ammonia, etc. or other minerals that aren't found in natural sea water. Most of this is okay for the fish and corals, but can feed algae in your tank.
I would somehow feel not so good using water that contained postassium for my aquarium, but postassium is found in small amounts in natural sea water, and I have no clue what an overabundance of it would do to your tank.
The salt softener, I agree with Scott, should be just fine to use.
Sorry that's not really a definitive answer.
welcome to AA, btw.
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