dead shark fish

Question
It has been a long time since my fresh water tank has been cleaned.  However, my shark did not seem to mind and did well in it.  I used to clean it a lot in the past but since my back injury I had to stop.  I decided to get it cleaned today. My husband used a little white vinegar to get out the hard water off the glass.  After putting back the shark in the water, it looked sick and died about two hours after.  I am really sad and confused.  I would really like to know what we did to cause his death.  He was about 5 years old.

Answer
It really depends how well you cleaned it, as it is possible to over-clean a tank.  It will cause the loss of beneficial bacteria and be more susceptible to spikes in ammonia and such.  This is most likely what occurred since the fish had come quite comfortable with the conditions it was in.  When cleaning an established aquarium, a little goes a long way: 25% water changes, gravel siphoning, and using a vinegar solution on the outside of the glass is more than adequate.