i used to store hot water in thermos that the inside glass got stain from hard water (apartment water was hard, contains a lot of mineral) after i got a son, w

i used to store hot water in thermos that the inside glass got stain from hard water (apartment water was hard, contains a lot of mineral) after i got a son, we replaced hard water with ro water, including the thermos. well, baby is important, right? over time (months), the stains are gone, the glass inside the thermos got cleaner. so yeah, ro water being soft water probably cancels out the stains from hard water previously used. on waste water for aquarium: it's because you misrepresent 'waste water'. water in an ro unit gets filtered four times (must go thru 4 different filters). the first is sediment, and the last is the ro membrane. can't remember the middles. the sediment filter gets brown over time and need to be replaced (cheap, ~$1 per cylinder). so there's some impurities captured. after last filter, the membrane, the water that fails to pass becomes 'waste water'. it's actually clear, tasteless, just didn't pass the ro membrane. so ro waste water is actually cleaner than the input water. my fishes are okay with water from well. naturally, they should be okay too with water that's cleaner than well water. on heavy metal, fishes are very sensitive to cu, not so much for pb. so if fishes are healthy and not jittery or die after a day, the water should be okay. i also bought ro unit for my soft-water fishes: neon tetra. accidently, i found that human can drink ro water too ;)