Yes, they do best with a water temp between 21 and 29C, so they are considered tropical fish.

Some people do keep them without heaters, but they either live in a warm climate or keep their heating turned up.

If they are kept in cooler water they become lethargic and prone to disease.

Ian

Edit: Heat lamps don't work well. Problem is that they aren't controlled, the tank might be too hot or too cold, and you need the heating most in the early morning hours when it's coldest. During the afternoon it's probably warm enough, and a proper heater will just switch itself off.

Bettas will live at room temperature but they are not happy. If it gets too cool below 70 degrees for too long they get sick and die, they become sluggish and usually get an infection. They need warm air above the surface of the water. I keep them in heated aquariums. If you must keep them in a room without a heater choose a warm place the closer they are to 80 degrees or a little above that the happier they are. Make sure the water is always exactly the same temperature during changes. Remove chlorine from new water. I keep them in warm rooms of my house without heaters with a large vase and bamboo plants. I do not change the water often the plants filter it. Lots of light for the plants is needed. Sick bettas do better on Kordon Rid Fungus it also works on Ich and is herbal. ebetta.com is a great online website for help. And don't forget in aquariums - pets question and answers,www.5d2d.com

The Importance of Cycling an Aquarium and only doing small water changes so that you do not upset the biological balance:
I have raised fish for more than 35 years, the most common hardship that people face when raising pet fish or pond fish is due to not understanding the importance of the Nitrogen Cycle and Cycling the aquarium or fish pond. It is extremely important to gradually add fish that get along to any aquarium. If you just did more than a 50 percent water change then the aquarium or pond needs to be cycled again and the water quality parameters of Ammonia and Nitrite monitored. Never change the pH suddenly it is fatal or near fatal. Always remove the chlorine,chloramine and add water that is the exact same temperature so that there are no sudden temperature changes on the fish.

Monthly water changes of 10 to 20 percent are recommended, in some instances it should be done weekly. Always monitor the Ammonia and Nitrogen levels and add beneficial bacterial enzymes if needed, usually they are needed and can be purchased. BIG WATER Changes of over 30 percent can actually do more harm then good, and upset the balance.Overcleaning of the gravel and tank is a fast way to kill off the fish you have raised for so long.The beneficial bacterial colonies live on the surfaces of the filer material,gravel,rocks,plants,etc!

Green algae means that the water quality is in balance. Green algae looks bad but is beneficial. Reduce the lighting number of hours and keep away from a bright window.Reduce the lighting to around 8 hours daily;use a coffee pot timer if needed. Algae will first occur as a brownish film then gradually green up, at that time you can also add an algae eater or snails of appropriate type. Do not kill off your pleco or otocinclus by putting it in the tank too soon!

When you are ready I suggest a heated smaller aquarium as a quarantine tank for new arrivals for two weeks, then add them to your main aqaurium. Water should be cycled in both aquariums. I also highly recommend sponge filters maintain beneficial bacterial colonies. New fish should be monitored for possible ick and fungal infections, these can be frequently seen in fish from large pet store chains and stores which sell fish and linens and more. Stores that sell clothing and fish etc tend to be clueless as to what they mix in the same tank. One of the worst examples of this was male fancy guppies in a tank full of spotted green puffers. Not only do those places have low quality fish care but they are expensive. pets question and answers,www.5d2d.com

My favorite products are Aquarium Salt (non Iodine salt used for water softeners,ice cream making or sold for freshwater fish),Kordon Rid Fungus which also works great on Ich. Low level doses of Malachite green. Scaleless fish like tetras,catfish,ghosts,silver dollars, loaches,botias should be treated with scaleless type medication or reduced levels. Do not use copper products on snails,shrimp,or crustaceans.Antibiotics should only be used when there is a bare bottom aquarium and the water quality is being checked. Many bad water quality issues can trigger bacterial and fungal issues, and antibiotics are not always beneficial. They tend to also destroy the water quality.

Increase oxygen if water is above 80 degrees Farenheit, and always increase oxygen when treating sick fish.