Without knowing your parameters, its very difficult to say if your water is the cause or if its a parasite issue. Its not a fungal issue. Drastic swings in ph are a possible cause but as we dont know what your ph was in the past or what it is now for comparison. Perhaps your municipality switched to using chloramine or increased the level of disinfectant they are using and now you have a spike in toxins. A drop in temp as well may have been enough to stress your fish and lower their immunity to a parasite that they were already infected with.
It really is vital to know your water parameters and what is contained in your source water, especially when you have sick fish because the majority of fish illness can be directly or indirectly related to issues with the water.