pica_nuttalli
don't be a twit
You folks that heat your tank with the light: What do you do, keep the light on 24/7? The small tank will not hold the heat at night, so the fish will experience large temperature fluctuations, which are not good. No heater? Heat the room or get a waterproof heating pad to put under it. 50-60 degrees is way too cold for Betta. The fish needs his sleep too, so don't keep the light on all the time. He can't close his eyes--let alone the algae growth.
actually, you'd be surprised how well 5g of water will hold heat. my dad's 5g hex stayed in the 76+ even overnight. i will concede that there was about 4 deg of fluctuation on a daily basis, but it was all within the acceptable tropical range. there's debate about whether that type of temperature fluctuation is harmful.
but you're right about one thing: anything under 73 degrees is too cold. so everybody who keeps their betta sans heater needs to have a thermometer in that tank.
If you must buy your water, buy RO water from the lfs. Otherwise I would think it would be better to use water from the same source, like the tap, so the fish doesn't experience changes in the water composition + all the other warnings given.
RO (reverse osmosis) is only a good choice if you replace the missing trace elements with a powder. more trouble than its worth, in most cases.