Betta not eating

[Kathie]

Hello everyone! I am a new "mom" to a betta. He currently resides in a 5 gallon tank with a filter. Since bringing him home three days ago, he hasn't eatten anything. He can usually be found behind the filter or at the top of the water. He will watch the food, approach it, then slowly swim away from it. The woman at the pet store said that being that I had a plastic tank, I shouldn't use a heater, but that the fish would be fine. Could it be the temperature? She also told me that I didn't need PH water testing kit. Just use the water conditioners at every water change. Could it be the ph level that's throwing this poor fish off? Could it be too cold? Is he just getting aclimated? Please help! I'll do whatever or buy whatever is necessary.