black moor fish keep dying overnight

Question
QUESTION: Bought a 25 litre tank for my 8 year old son.
Have filled with water, added conditioner and left with filter running for a week before we bought the fish.
We did the routine of climatising before adding to the tank etc and after a good 30 mins, released into the water.
7 hrs later dead fish.
Did ammonia and nitrate test and all was good (ammonia slightly high but caused by the dead fish itself)
I changed 20% of the water, added conditioner etc and tested the water again this time at the pet shop. Again all was ok.
Same routine etc in the morning dead fish again.

Any thoughts before we try again?

ANSWER: Thats crazy!! Especially since the water quality is testing great?!! Theres no other explanation except that whatever water conditioner you are using isnt removing the chlorine from the water and the tests didnt catch it. The other thing that could shock them to death literally is the instant ph change, if there is one. The pet stores normally try to keep there ph all at the same levels for every fish that is in there. What people fail to realize is that a lot of the fish are native to ph levels different than what they are currently living in. Fish farms and captive breeding make them more hardy.  What im trying to get at is, if the ph level is drastically different at the pet store than it is in the aquarium, then it is possible that it could be to much for the fish to handle. Sorry i couldnt be of more help. Good luck.

---------- FOLLOW-UP ----------

QUESTION: The store have recommended we completely flush the tank and start again in 5 days. I will take another sample and suggest a ph test as well and see what they say.
Thank you for your quick response.
The annoying thing is we have had fish before and never had a problem. Maybe its the quality of theoir fish stock?

Answer
And that could very well be too. Not too long ago, I started up another 55 gallon tank in my home, and I cannot tell you how many angelfish i went through. Everything was right for them to thrive but, they would die off within a few days. I questioned the place and to my amazement, they admitted to not being able to keep the angelfish alive in their own hands. Most of the fish come from a farm. If there is something wrong or different from the farm to the pet store, theres the problem. I eventually found a family owned pet shop. It was further away from me, a little more pricey, but the quality and selection of the fish species was astonishing. Try to find a small local pet shop that isnt a franchise store. I wish i could help. IF i may be of assistance to you in the future, feel free to contact me anytime.