Betta Sick?! EEK! Please dont die!

Question
I got my boyfriend a betta fish about 3 months ago and he looked to be doing ok. Well now I have him because he is going  on a vacation with his family for a bit and he looks to not be doing so well. My boyfriend neglected to clean his tank for so long that the water began to smell!! I couldn't believe he did that. But now he seems to not have so much energy, some what just floats around... sinks to the bottom often even when I cleaned his tank its a gallon at most and gave him 100% fresh water. He is a white fish with some red and purple so it's hard to tell if some of the color doesn't look right.
Then when I feed him he just lets it sit up top and eats maybe 1 or 2 every feeding.

I know something just doesn't feel or look right about him.

Answer
Hi Kelsey,
 Do you have a heater in the tank?  Bettas are tropical fish and if you don't have a heater that water will be very cold now and so the betta basically shuts down.  His water should be around 75-80oF.  Also, you should be changing 25% of his water once a week, every week. That is actually better than changing all of his water at once, which can
be quite a shock.

-- Ron
  rcoleman@cichlidresearch.com.
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