hard water is ok. if he is a crowntail, it can thin and/or curl his rays, but its not unhealthy for him.
RODI is no good. it needs some minerals in it.
but what i do is mix my hard tap water with RODI to make soft water. that is the best solution. try a 1:1 mix.
1 gal is ok. just is gonna need daily 50% or more WCs. the prime used for the water change should bind up any ammonia made until the next one. heat is important. 76-82F.
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