Question
Raf,  I need your help.  We have a backyard pond approximately 350 gallons.  It has a vinyl liner, a Laguna 700 filter with UV lamp and Laguna 1500 pump.  The water enters the pond as a fountain and elevated hose to provide aeration.  There is a water plant to provide shade.  We had 2 goldfish and 1 Koi  about 6 inches long and 6 mosquito fish living great for 1 year.  The pond became very green suddenly and the fish sadly died.  It was my fault as the uv light had burned out for some time and my misunderstanding of removing the filters to clean and not just back washing and using the built in scrubbing brush feature.  We have refurbished the unit with new bulb and filters.  Refilled the pond, adding the chlorine remover, liquid beneficial bacteria, and it tests ideal on the test stips for PH, alkalinity, nitrates, nitrite, and hardness is a little high.  We let it condition for two weeks.  The temp is around 75 degrees.  We placed 12 small mosquito fish and they all died within 3 hours.  We were very confused and about two weeks later took a water sample to our local pond and fish store.  They tested it and it showed everything perfect.  So we bought 12 more mosquito fish and placed them in the pond letting them acclimate for 20 minutes.  They all died overnight.  What are we missing here?  They seem to be having plenty of oxygen from the aeration.  What do you recommend?

Answer
Dear Paul,

Thank you for your question and sorry to hear it's not going well to add new fish.
mosquito fish are normally able to live even in water with low oxygen, so I think there should not be the problem.
Directly I can think of some items to check.
As in fact the water is complete new, maybe the water values like Ph are fluctuating during the day/night cyclus. You need to check this 3 or 4 times (morning, mid, evening, ,night#.
If it fluctuates more then 1?there can be a issue.
Also you can try to check the water quality in the sack to check if this is approximately equal to the pond itself.
My experience tells me that when fish die within a range of few hours it tends to be #in many cases# something like this.
To extreme difference in water quality or temperature
Sick fish
To much stress.

In short my advice would be to check water quality over 2 days and this if possible approximate 4 times in the 24 hour cyclus to start #mainly Ph, GH, KH ammonia, Nitrate#.
When fluctuating more than 1 degree in Ph, adjust values (hardness adjustments to stabilize Ph).
Is buying new fish, when home check water quality in the sac and add every 15 minutes some pond water in the sac and place it on the pond surface to float.
After approximate 1 hour the temperature should be the same and also water quality should be more even.
Make sure the pond is aerated, however try to leave a area without extreme water movements to let the fish go hide for some time.
Add the new fish in the afternoon if temperatures can be high or fluctuate extremely.
In addition you can try to add some pond salt as this often reduces some stress.

Hope this helps and good luck!!!

Kind regards,
Raf