i take about a gallon out every two weeks. becuase the nano cube has such a tight fiting lid water can't evaporate like it would in a normal tank. so in a reagular tank when you top it off you are adding fresh water and all the other good stuff that everything needs everyday or so. in the cube the water pretty much stays there so it is even more important to do your water changes.

i also recomend macro for the nano cube. there is alot of light in there and the lights keep it pretty hot so by adding marco you are feeding it instied of the bad algae.