Unusal Behavior

Question
My Cichlid's are staying at the top of the water as though there trying to get air.  They just started doing this since I change a 1/3 of the water.  Why are they doing this.  A test of the water shows everything OK.

Answer
hey art,
do you have a heater in your tank?? this is the only thing i can thing of as the answer. I have seen and heard of many fish tanks with tropical fish with out heaters and the fish swim at the top. also, if the temp isnt up high enough, they could do this.  it needs to be between 75 and 80 degrees.
i hope this info helps, if this isnt it im not surewhat it could be because i dont know about medical issues that could cause this.
best wishes, chelsey