Since doing 25% water change and adding medication for fungus fish have become inactive at top of tank. why?

One fish was very healthy before but the other has had a white spot like cotton wool on dorsal fin for the past 3 days ad was lethargic from time. Also turned heat down to 18 degrees C as was at 24 and thought this might be too hot and stressing the fish. Had to remove the filter also. Any ideas as what is wrong and how can I can solve the problem. Tank is 13 gallon and set up 4 weeks ago with fish added 3 weeks ago Since doing 25% water change and adding medication for fungus fish have become inactive at top of tank. why?Yea, go buy some conditioner. That water probably had chlorine so their gills are burning. You have to condition your water. Hope I helped!