The thing about water softeners is that they tend to decrease the amount of chemicals in the water, some of which are needed for fish to survive. The way that most water softeners work is that they replace the calcium content in the water with sodium content. Certain fish do not do well with high sodium content in the water, and in those cases water softeners would not be a good option to use.
Like Effin &Jeffin said, if it's a salt system, may not be great but many softeners are not salt systems these days and in that case it would be fine.
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