HONESTLY the girl @ my pet store gave me the same adivce, so i had my betta in a half gallon tank with no heater, feeding him once a day ( the equiv to one pellet)
(1 100% water change a week)
After about 6mnths, my betta was pale grey, lathargic, fin rott, very tiny...and just very sickly. After joining this forum i decided to move him into a 5gal with heater, doing 100% water changes every week and 15% daily. And feeding him twice a day
After about 1 mnth he is starting to get his colour back, fins are re-growing and i havnt seen him so active since i first moved him out of his cup when i bought him!
DEFINATELY happier and removed from the jaws of death! LOL For sure under the old conditions freddy wouldnt have made it much longer!
THANKS to all you guys/girls on the forum who help people like me give better homes to their bettas! :) :)