koi staying in shallow water

Question
out of all my fish I have one that has been staying in shallow water with its back out of water.. If I touch it he swims away like normal. I have tested the water, where it is only one I was wondering if it is sick ???

Answer
It's possible that your koi is stressed, especially if there is a lot of direct sunlight or ambient heat.  As the water temperature increases, the amount of available oxygen decreases.  The shallow area probably has more has a higher dissolved oxygen content, especially if there is water turbulence to prompt beneficial gas exchange.

The quickest way to eliminate this problem would be to remove the majority of any leaves, dying plants, and dead algae.  All of these are considered organic waste and when the naturally occurring bacteria aid in decomposing them, your water may experience a drop in available oxygen for your fish.  You may also consider slightly altering the way the water re-enters your pond.  Try and break the surface of the water as much as you can.  Adding shade-providing plants goes a long way as well.

For this particular case, observe your fish's behavior.  If he/she swims away fairly quickly when you touch him, I wouldn't worry too much.  If this behavior is prolonged or he/she appears to be gasping at the surface, it is a sign that immediate attention is required.  Keep in mind that a fish may be struggling to breath for reasons other than oxygen deprivation.  It could be due to an excess of a chemical compound, such as ammonia.

In the future, you can keep an eye out for ulcers, difficulty breathing, and swelling of the midsection.  These symptoms are highly indicative of stressful water conditions and may spread to other fish.