Reddit mentions of Cobalt Aquatics Flat Neo-Therm Heater with Adjustable Thermostat (Fully-Submersible, Shatterproof Design) from 25W to 300W

If it was me, I would clear out the whole tank, wash it out with vinegar and rinse the substrate really well. Not much else you can do really....

You may want to invest in a temperature controller. They are well worth it. I had a heater fry and luckily I had a controller that shut everything off. This is the one I currently use.

Alternatively, I have one of these on one of my tanks. It advertises that it is shatterproof and includes an internal shutoff to prevent overheating, but have not had the misfortune of having to test it (yet. knock on wood).

Anything 10-25w should be sufficient, but definitely do your homework on them as a lot of the inexpensive heaters have a good chunk of people complaining about malfunctions.

I've recently used:
http://www.amazon.com/Cobalt-Aquatics-Neo-Therm-Heaters-Plastic/dp/B008AGHH8Y/ref=sr_1_1?ie=UTF8&qid=1415980376&sr=8-1&keywords=Cobalt+25w+heater

Large for smaller setups, but very accurate and stable; reviews do it justice.

http://www.amazon.com/50-Watt-Titanium-Aquarium-Heater/dp/B008OTJEGA/ref=sr_1_4?ie=UTF8&qid=1415980631&sr=8-4&keywords=50+Watt+Titanium+Aquarium+Heater

This one I liked because it's small and fits well in the pump section of my Fluval Spec V; I ended up giving it to my roommate as their 3.5 needed something smaller than the Cobalt above and it works just fine.

Expensive I know, but with all the 10-15 dollar heaters either frying of freezing fish I didn't want to skimp on one. I do have a marina 10w in the tank with my guy right now that's not pulling its weight so it gets swapped out today.

Edit - repeating myself.

I would definitely cycle the tank...it'll save you a lot of work later on. I think it's safe to say that it's absolutely essential for having a successful, healthy aquarium.

As far as heaters go, I recommend the cobalt NeoTherm 25watt heater. It's on the pricier side but it is adjustable and accurate, safe. I use it to keep my 2.6 gallon tank at 78, it is super accurate. Very high ratings. Doesn't get dangerously hot or anything.

Cobalt Aquatics Neo-Therm Heaters with Plastic LED, 25-Watt
https://www.amazon.com/dp/B008AGHH8Y/ref=cm_sw_r_awd_-MaAub0X0HE8C

Ha, love the name idea!

I'm not sure if you have read around and are aware of the proper care for a betta (if you have then ignore me) but if you haven't I'll go ahead and say a two gallon bowl isn't the best place for him, and is also another reason for the downvotes. He can live in it but he's not going to thrive. He does need a heater and a filter, if you have those then that is awesome, if not you can even get them as a broke college student.

If I can tell you one thing I'm positively sure of, there is always room for a fish tank. ;)

I'm just going to link you a cheap list of good stuff for little moneys:

Sponge Filter

Air Pump

Airline Tubing I bet you can find this in stores for less. Also less as an add on item.

Check Valve Most likely less at a store.

Heater, Heater 2 Costs a good bit more but I really love these.

Tank, Tank 2 There are many other options to look at.

If you go with the cheapest it will run you just about $58 with prime.

Hope the list gives you something to think about. If you've got any questions at all I'd be more than happy to answer them if I can.

Edit* Added a link.

I have the same tank! I actually just rinse off the one I have in old tank water every time I do a water change (not tap water, it will kill the bacteria). I never change the cartridge.

I also have this heater. It is a little pricey, but I used to have a cheaper one and the temperature was not adjustable (preset to 78) or stable, it would drop during the night. This heater is great, you can pick the temperature and it stays constant.

Little to tall, found this one looking at that one though. Pricey, but thin and short.
https://www.amazon.com/dp/B008AGHH8Y/

This is what I use in my 5.5gal betta tank: https://www.amazon.com/Cobalt-Aquatics-31000-Neo-Therm-Heater/dp/B008AGHH8Y

So far so good!

Some options (but do check reviews, obviously you want to see more good/positive reviews, and minimal bad/negative reviews)

https://www.amazon.com/dp/B008AGHH8Y/

https://modestfish.com/best-aquarium-heaters/

Here's a great filter and here's a great heater for that tank. I would try changing 0.5 - 1 gallon of water once a week once your tank is cycled. Keep an eye on the test results and adjust your water change routine based on what keeps your levels low enough. Make sure you add water treatment (Prime) to that tap water before you add it to the tank.

Bettas prefer should be kept in temperatures of 76-82 degrees F

Recommends 78+ [/r/bettafish caresheet] (https://www.reddit.com/r/bettafish/comments/3ow6vz/info_betta_care_sheet/)

While they can survive at 72-86 the ideal range is 76-82 bettacare101

Edit: Most likely the heaters you tried to use didn't have enough power. For a 5 gallon tank a 25 watt heater would work. Heaters that you can sent the temperature of work much better. Example

I'm still new but this is what I have read:

I'm getting this heater for my betta. Although this heater is cheaper and has some decent reviews; link also lists other simular ones. I'm getting the more expesive one listed in the first link, cause I like that it atcts like a heater and thermomiter.

As far has plants go, if you get faux, make sure they are silk and don't have any sharp or rough edges on them or it could tear your fishes fin. I have read that one good test is to take the plants and decorations and run them through a panty hoe. If it doesn't cuase a run, it's safe. Live ones are better since they help get rid of toxins. Having a 1-3 real Marimo balls are always good. If your new to tank keeping I would search for plants that work well in low light, and can handle brackish water incase you need to do a salt treatment.

Get the API Fresh waster master water tester. Strips are ok, but not uber accurate sometimes and can end up costing more money.

Seen some people say they like the Fluval tanks, just have to adjust the filter so the current isn't too strong for the betta and might have to mod it to get a heater in the filter bit. I currently have a 2 gallon, but eventually would like to get him in a 5 gallon. I would not go bellow 2- 2.5 gallon.

I give my betta fish frozen brine shrimp; just one a day. Some people do the flakes and pellets, but frozen food is better. you have to be careful not to over feed them, or they will bloat and get swim bladder issues. Feeding them a blanched deskind peice of pea helps keep them regular.

Again, i'm still really new to betta fish, but this is what I have read online. The sticky INFO: Betta Care Sheet has a lot of good info to read.

I have a used version of the Cobalt Neo-Therm heater in my 5g. I like the smaller size and the temperature indicator and adjuster makes it easier to set to the temperature I want. Currently there are a couple used $30 ones:

https://www.amazon.com/gp/product/B008AGHH8Y/

I've got one of these 25 watt Neo Therm heaters https://www.amazon.com/gp/aw/d/B008AGHH8Y/ref=yo_ii_img?ie=UTF8&psc=1 which is a bit pricey but I've been pleased with it. It's a bit wide but flat so pretty low profile overall. I've never had any problem with it. (The picture on the product page seems to be a higher wattage model that's longer, the 25 watt is less than 8 inches)

Here is the mobile version of your link

What heaters are these? If they’re heaters with a present temp they’re really not reliable. I have been using the 25W Cobalt Aquatics NEO-Therm on my 4.6 gallon and it does a good enough job. I’ve set it to 78 F and it fluctuates between 76 and 79 F.

Well, $42 I think on amazon. But, sometimes I've seen them cheaper. But, have to be honest, depends on what you have in the tank, where it's located in the house, and what the house ambient temp is year round.

So, I have three 5.5 gallon tanks:

One has an Aqueon 50 watt preset heater with shrimp in it, heats low to mid 70s pretty much year round, ok for shrimp, not so much for fish. This is on an outside wall, so the 72F in winter is something I have to watch out for. If it's an exceptionally cold winter/cold snap, I'll add a Cobalt Aquatics 5 watt for a boost. And since I have glass lids, the lights do generate some heat if they lay on the glass vs raised off the glass w/those little "legs."

Another has the Cobalt Aquatics neotherm, it was stuck for well over a year at mid 70s because no matter what I did I couldn't adjust the 2 buttons at the same time, I called them they offered to send a new one for free...I didn't receive it as scheduled but I also didn't bother calling them back, I'm sure they would have resent. Just this year, I took the lid off, pulled the suction off the back glass, and grabbed that sucker with both hands and I guess I gave it the leverage needed to force the stuck button, so now it adjusts lol. my laziness really not wanting to do all that before now but the tank was planted and I didn't want to destroy everything doing that... now I'd been using it as quarantine, so not really anything much in there so no worries lol.

And the third one is also on an outside wall. This one has a 20 watt TopFin Betta preset heater AND a Cobalt Aquatics 5 watt preset and this winter when it was really cold, an Aqueon 10 watt preset. I can play mix and match like this because I have had quite a few little nano tanks that I've decommissioned so I can do that. Right now, it's 85F in the daytime so I've only got the 20 watt and the temp is 78F.

The only heater of the above ones I mentioned that was expensive was the Neotherm, so...you could go for a cheaper one.

ha! I decided to see if they had a 50 watt Aqueon PRO and they do!!! Says it's 9 inches high, so it's shorter! And less then the neotherm.

I have 2 Cobalt Neo-Therms at home. Highly recommended, just set it and forget it. It's a little more than you want to pay but worth it.