Arowana Only Survivor

My fish pond is toxic!  No goldfish or red fish survived more than a few days in the pond.  Only my two feet arowana can live in the water.  I wonder if the fortnightly fogging is poisoning the water in my pond.  At first, I thought the fishes from a different source is not good.  But when I go back to my regular supplier, same thing happens.  So it is not the fishes but the water.  I replenish my pond with all the rain water I can get and then I will try to introduce goldfishes again.