Heater Question, Please Help!

It's not necessary, but it will provide warmth to your betta which will raise its immune response and keep it happier. If you're in a tropical climate, (lets say, Philippines) you won't probably need a heater due to the warmth but you will if it's not stable. I have a highly recommended heater in my 3.5, it never overheats. It's called the Hydor THEO.

I also recommend Visi-Therms and Eheim Jager heaters.
Your betta will live a happier, healthier life with a heater. Get an adjustable one so you can be prepared for any needed increase or decrease.

Betta are tropical fish so yes having a heater is a good idea to keep the temp at the lowest to 76 for them to be healthy and warm. You can get small heaters for the smaller size tanks and bowls like a 25 watt. I have a 5 gallon and mine is a 50 watt Elite adjustable heater that I bought on Amazon which have very good prices on their heaters. You really have to be careful with the smaller tanks and bowls that the water does not over heat which you can do buy getting a thermometer and keeping and eye on the temp for a couple of days. I hope you find what you need.

technically, the heater costs more than the betta, so it's a toss up ; ) You can decide to have 10 bettas before you break even with the cost of a heater.

I have a 50W in my 2.5g QT tank that I moved from my 5g main tank to keep my Fred warm as he heals. I ordered a 50W for my main tank that has my amano shrimp and snail.

Last thing you want, and I learn from experinece, is to keep him in a small tank (less than 2.5g) with an unpredictable pre-set heater that fluctuates from 72 to 78F during the day. That costs money to but AQ salt, etc, and time with extra water changes.

To me the heater is worth it because it eliminates unnecessary stress for me to keep Fred healthy and alive.

I also did not have a heater for my first Betta because I didn't think he needed one but once my friend gave me her old preset heater my Betta was so much happier! They can survive without one but their standard of living goes WAY up with one. I have thermometers to make sure that the tank with a preset heater stays stable. So far so good

I would suggest the getting the heater. The lowest water temperature for them is 76F, not 70F. Sure, they can survive in a colder environment, but once you get a heater and have the water at around 78-79F, your betta will be much happier and much more active.

Tanks smaller than a 5 gallon are heatable. A 25 watt heater can heat a 2 gallon tank, as long as it's adjustable. If you change to a 10 gal, get him a 50watt.

I actually ended up with a 5.5 gallon tank because my mom didn't think a 10 gallon was necessary, how many watts would that be? would that be a 25?

I personally like to have a heater in my tank, just in case. You never know. It is always better to be prepared.

25-50watts are fine. If you think that you're going to get a 10gal someday, get a 50watt so you can still use it after the upgrade; it's fine as long as it's adjustable.

25 watts will be great.