are you useing tap water,if so you may want to check and see if there is cloramine in your water,it does not evaperate out like chlorine does,I have it in my water and it took me a while to figure out what was causeing my problems,someone told me that a product called amquel not sure on the spelling on that but you can get it at pet stores its used to treat tap water for fish,I just decided to go with rain water a little more work but its worth it