Quote Originally Posted by Mentat View Post
5 months without a water change on a small tank with a goldfish will have Nitrate levels so high it will affect the fish... and that's on a cycled tank with 0 levels of Ammonia or Nitrites !
I guess really it depends on how many water plants you have and how much water you replace when it evaporates. I have natural tanks with plants, very few creatures, and no filtration and they can last a whole year without a water change.