I guess it depends on what part of the country you are in and the temperature fluctuations that can happen because of it. If you live in a place that has warm temperate days and nights this might not be such a problem but if you live where I do and it is only warm a few months out of the year, that is a different story. Some people think that they know everything in spite of thier inexperience. She said she has been keeping fish for a year? That is hardly experience! I have been keeping tropical freshwater fish for more than ten years now and recently; nine months ago have gotten into the salty side of it, and I can tell you I learn new things all the time in the fresh water and the salt water part of it. Some people are just too full of themselves and think that they know everthing about everything. Don't let it get you down. You did the right thing to tell her what you did. Next time, if she comes in the store and insists on buying those kind of fish for her unheated tank, tell her that you can not guarentee her fish and make sure that you write it on her receipt.
Marinemom
Marinemom