The water is hard, pH 6.8, no ammonia, nitrite and nitrate are fine.
I don't know exact details but i got my water tested by a trusted store and it was all fine as usual.
I had my water tested from a trusted store. They told me it was safe...oh, how it wasn't. Get a test kit!
We have read so many times of a store saying the water is 'fine' when it is not fine. Did you get numbers for the tests? If not, always insist on the numbers. The only 'fine' numbers are zero ammonia and nitrite and nitrate under 20 ppm.
You fish problems could be due to the store misleading you about ammonia or nitrite levels. Once you can test for yourself you'll be much better able to decide if the water is the problem.