I can’t imagine a church actually putting bleach in the holy water! I agree with the others that it may, indeed, be the tap water. I live in Chicago and there are times when the water has an incredibly strong odor! Among my many plants, I have orchids, and I used to let the tap water stand for several days before I use it on them. (I feel sorry for the person that lost aquarium fish.) I then started buying distilled water from the pharmacy and finally invested in a still. I also have several indoor fountains and the other stuff in the water would eventually form what is known as “boiler scale” (which is formed of calcium compounds in the water). The distilled water completely alleviates this problem.
As to your specific question, I would suggest you speak with the sacristan or a parish priest to try to find out what is going on. Hope this helps!
Pax tecum!