New Pond Owners, Same Old Pond

Question
We recently moved into a new house, and a pond was already in existence.  Apparently the pond had been kept for years and had many plants and fish living happily.  The house had been empty over the winter months, and the sellers gave all their fish away for friends to care for.  Or, they THOUGHT they gave all their fish away.  We bought the house in early spring, and the ice on the pond started to melt.  After a month, we found a single living fish.  We also found potted plants left in the pond, which were still alive and bloomed with the warmer weather.

If there had not been a living fish in the pond, we would have emptied it out and started with fresh water.  Then we would have treated the water and tried to introduce a few fish.  Because there was a living fish, we assumed the pond had developed healthy bacteria and we didn't want to start over completely.

The fish was a small orange koi, and after two months it had doubled in size.  We were feeding it flakes and it also liked to snack on algae on the side of the pond. Because the fish grew so big, we assumed it was healthy and it was an appropriate time to add more fish.

Our koi started breathing at the surface a bit, but we were advised that the addition of more fish would actually help the oxygen levels in the water.  We were told that the additional fish would eat the algae and that oxygenation would improve. So, yesterday we bought two more koi fish and introduced them to the pond. They seem to be getting along fine with our older fish, but I am seriously concerned about the health of all of them! All three seem to be constantly breathing at the surface.  What could cause this problem?  I do not want them to suffocate!

We are new pond owners, but we're keeping an old pond. We're clueless. Please help!

Answer
I would have put that fish in quarantine and cleaned out the pond when you had the chance.  Adding fish does not in any way improve oxygen levels!  Algae produce oxygen during the day so if fish eat it, there is less oxygen during the day.  However, algae does use oxygen at night.

I'm betting it's really warm there now, right?  I would get an aerator and get that water aerated well.  You can buy an aquarium pump at most fish stores but if your pond is over 500 gallons, you'll need something stronger.  What kind of filtration and aeration does the pond have now?  I've lost half a dozen goldfish the last few weeks since its been in the 90's and that's with two separate pump/waterfall systems and an aerator!  I also suggest doing a water change (add dechlorinator afterwards) to cool down the water.  If the pond has a lot of algae, that could lower the oxygen levels overnight making gasping occur in the morning.  Get that water moving!

As for algae control, see http://www.fishpondinfo.com/plants/algae2.htm

Good luck!