They are bothered by the chlorine. That is why when I go to refill my tanks I do not do it all at once. I add 15-20 gallons and wait 30 minutes. I never have a problem and I use no chemicals.
U may think u use NO chemicals but ask the water company how much they put in ur tap when treating...... they have to kill pests and micro organisms. Prime takes these out and makes the water safe! Unless u have well water use a good dechlor. This was step 2 of fishkeeping ppl.
#1 S. Vettel