"Inabón Yunes" wrote in
:

Distilling water is not such a good idea. I used distilled water in
my tank for two months and my KH dropped like crazy. See, distilled
water has no KH.
To tell you the truth, I save tap water and sit it for a week. My
filter takes care of stabilizing it but I keep the new addition to no
more than a gallon every two weeks.
Remember, I don't have a reef aquarium, corals are very sensitive to
chlorine, even a small amount will stress the polyps not to mention
anemones.
iy

Remember, though. The OP was about what to do with _used_ water changes
taken out of the aquarium, in an effort to conserve water;

"Is there anything I could do with water change water from a marine
setup... or is it a case of tip it down the sink.(?)"

Presumably, the OP will be getting his original mix water from a source
that he finds to be acceptable, and is looking for a way to make salt water
useful after it has served its function in the aquarium.

I'm with Frank, keep the used marine water in a bucket by the loo, and use
it to flush the toilets, in place of city/well water.

DaveZ
Atom Weaver