Cichlid Constantly Swimming To Surface

Question
QUESTION: I recently had a 2in cichlid die for no apparent reason. For two days, it maintained a constant vertical swimming position as if it were trying to reach the top of the tank. It refused to eat. The other cichlids in the tank are fine as is the 4in Gourami. Any insight into the cause and how to prevent it from happening again? Thanks, Jean-Paul
ANSWER: What are your water parameters? Ammonia, nitrites and nitrates?

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QUESTION: I'm actually not sure. I had assumed everything was fine since the other fish have been ok for more than a year and I have a robust filtration system and change water regularly. Does the constant vertical swimming toward the surface behavior indicate a chemical imbalance in the water that only certain fish might be sensitive to?

Answer
Some fish are more sensitive to water toxins then others.  If they were head down it would indicate swim bladder disease, but what you are describing does not sound like a bacterial or parasite infection.  Check you PH as well.  Sometimes elevated PH levels can induce abnormal behavior.