Highya,VxLoNERxV....We have a 70 gal. fresh water tank and I'v been wondering the same thing for quite some time.
We do a 20% water change every other week and it just pains the hell out of me to just throw that water out onto the yard.
I've got a few seedlings started...Think I'm gonna save a gallon next change and just give it a try.
Might cut the tank water by about 1/3 to start and just see what happens....I'll keep Ya posted!........J