If your fish hang out at the bottom of the pond, or act restless
and lazy, then it's likely that your pond is suffering from poor
aeration.

When oxygen levels drop below 7 PPM it not only affects the
ability of your fish to breathe, but it also negates your pond's
bio filtration systems. That's because optimum bio filtration
results require large quantities of air. When bio filtration
degrades it results in the buildup of toxic ammonia levels. This,
in return, causes stress in your fish which can result in
lethargy and even death.

Your immediate first aid calls for increasing oxygen levels at
once. Unlike chemical treatments, you can't cause any problems by
putting "too much" oxygen into your pond. The easiest way to
maintain oxygen levels is to run an aeration pump 24 hours a day.
If you are already doing that, and your oxygen levels are still
too low, then either your pump is defective, undersized for the
pond, or your water temperatures are so high that the water is
not capable of retaining sufficient oxygen. Treat whichever of
these conditions exist.

Once you have cured the oxygen generation system problems, its
time to test your water for ammonia, nitrite, and pH. It's also a
good idea to test for total alkalinity and hardness. Adjust any
levels that are out of normal ranges using pond additives that
are designed for each problem.

If you found multiple problems, or suspect that you have a
general water quality issue, then you should commence a daily
changing of 20-40% of your pond's total volume until you have
turned the water over twice. Of course, you need to use
chlorine-free water. After you have turned the water over twice
you should perform all of the tests that I wrote about earlier
and make any required adjustments.

If your fish do not recover, or you are experiencing kills, then
you may have a parasite or other health problem. Check our
article archives for potential solutions.