Water that's been through a water softener can be a problem for some plants because of the salt in it, I think that's what Ken was thinking. Some plants are more tolerant of salt than others and it also depends how much salt remains in the water. I imagine your filter doesn't use salt so shouldn't be a problem. We have "hard" water here but I don't bother filtering it for plants. It supplies them with calcium and magnesium which they need and which aren't generally in fertilizers (but are usually added to commercial growing mixes as dolomitic limestone, which lasts for quite a while). Not saying that excessive hard water minerals can't be a problem for some plants, but I would expect them to show signs on the leaves first rather than suddenly dying.
