hi how often should you change the water in an octo tank. i heard once a month is fine but need a second opinion on that one especially how often would you have to change the water if the octo is in a 24gal nano aquarium with a 290gph.

In that small of a tank, you are looking at at least once a week...which is one of the reasons most of us prefer a larger tank, it gives you some more breathing room...still, you need to do water changes around twice a month or more.
greg

I agree with Greg, your tank is on the small side (most probably too small for a ceph!) and about 25% per week w/c