FW tank waste water is most excellent for plants!! It is generally high in nitrate, which is fertilizer, and tank water is better than Miracle-Gro, IMO. You can tell the difference in your houseplants.
I usualy do a 50% change on my FW
tank once a month. That will give me ten or so gallons.
what is funny to me is, I sit and pull my hair out over my SW
tank, and think, GOSH is it time again to clean the tank? But with my FW
tank I can literaly Ignore the tank, besides feeding, and never have to worry about anything but feeding the fish (which is 5 years old in this very tank) and change water.