ok nevermind about the light fixture I mentioned, its for a terrarium. I have all my reptile and fish stuff in the same room. If I decide to get anything other than fish I will look into new lighting.

As far as evaporation I can imagine that 1/2 a gallon would make a huge difference. I was just wondering about how much can evaporate in a day. I dont have a problem with topping it off every day. Maybe I will set it up with tap water for a week or two and see how much evapoation I get. That way I can mark my levels to know how much to remove when doing water changes also.