Originally Posted by Campbell
I have two unheated tanks, and unless your home is extremely cold it wouldn't effect a fish that much, IMO. .
Although I agree that there are likely other elements at work in this case, I don't think it's fair or proper to completely glaze over the importance of heat to a tropical fish. Bettas rely upon temperature for their entire metabolism--often room temperature is cold enough to cause digestive stress, lethargy, and compromise their immune systems. I would never advocate keeping a betta without a heater--especially when you don't know the living conditions of the person you're talking to. Even when your house is typically warm, you never know when you will face a sudden cold spell, or someone in your house decides they're hot and kicks up the climate control. Heaters are a very cheap necessity, they are a good investment.
As for the topic at hand, you should be doing 100% water changes on the tank that you have every day. Instead of betta H20, I highly recommend using tap water along with dechlorinator. The pet store makes more money from selling you water than they do from you making your own with dechlorinator--you will save money this way and your betta will have a healthier and more stable environment. Bottled water lacks essential minerals for stabilizing pH, among other things.
If you can't afford to upgrade your betta to a tank that can be heated, I suggest going to walmart or target and buying a rubbermaid/sterilite plastic storage bin. 4 gallon bins are only about $3, and can be heated safely. They're not that pretty, but until you can upgrade, it is a great way to house your betta.
I use and recommend this heater: http://www.fosterandsmithaquatics.co...8&pcatid=11368
it is important that the heater you buy has an adjustable temperature dial. This means that the heater will be able to sense when the water is cold and turn on, and sense when the water is too hot and turn off. Other pre-set heaters and heater pads cannot do this, and often overheat the water or don't heat it enough.