City of Astoria Water Quality Report
I doubt they are adding something new (having worked at a drinking water facility myself)
but seasonal changes can effect the mineral, or other components of the natural water.
If lead pipes are predominant in your area, phosphates are sometimes added to keep lead from leaching into drinking water (phosphate coats the inside of pipes providing a barrier to lead leaching out)
If the fish you added, are larger than you normally kept, their extra bio load may have instigated a bacterial bloom, and it would be temporary.
Your filters would not have an effect, because e bacteria is too small to be filtered out, and you just need to let the tank come to equilibrium, could take a month.
What is the volume of your water you remove with each water change?