Question
This morning we found two of our fish had died overnight in our pond.  We have approximately a 4,000 gallon concrete pond containing seven koi.  The pond has a really good filtration system with two combination bottom drain/aerators (that bubble up a lot). For the past couple of weeks we noticed that in the morning the fish were hanging out at the top of the water in the far corner of the pond near the skimmer.  There is healthy looking algae coating the sides of the pond with no string algae.  The pond is in a screen enclosure with two shade sails above.  The two fish that died were twenty inches long and had no visible signs of a physical problem other than being dead. The ammonia, nitrite, nitrates and pH tested fine as well as chlorine (we have a dechlorinator).  We don't have an oxygen meter and have since turned on the falls (we don't keep the falls on on it) all year due to the algae build up.  We had just done a water change, about 10% or so, yesterday.  Do you have any idea what could have caused the fish to die?

Answer
Dear Tom,

Sorry to hear this.
The only advice I can give is that there still could be a parasite issue (or bacteria).
To know for sure the best thing is to let a vet look with the microscope or call a hobbyist or dealer with a microscope to be sure.

Have you measured the PH in the morning afternoon and evening?
If there is a more than 1 point difference maybe the water needs to be adjusted.

Good luck!

Kind regards,
Raf