Hydor Betta Bowl Slim Fish Tank Heater, 7.5-watt, measures 3.5 inches round, black; For tanks 2-5 gallons

I originally got the 4w round heater for my Betta bowl, but it did not heat it to the temp I wanted (maybe a 5 degree increase), so I opted for the 7.5w which has been working wonderfully. I like the super slim shape which makes it easy to attach the heater to the back side of my Betta bowl behind the small filter that's also suction-cupped to the wall.The 7.5w heater has consistently kept the temp between about 77-81 degrees. My Betta seems happier and much more active and is eating more, so I believe the heater is making a positive improvement.There are no temperature controls for the heater, so if you want to be able to set a specific temperature, this may not be the heater for you. But if you are trying to increase the temperature by about 10 degrees in a small fish bowl or tank, this should work great.Some other reviews mentioned they'd gotten shocked when putting their hand in the water. That happened to me as a child when we had a really sketchy pump from a garage sale in our pond. So I stuck my hand in the Betta bowl to test and did not get shocked. The instructions do state that the heater should be unplugged before putting anything in there, but I wanted to note that the heater is well-made and designed to be submerged so it probably won't shock you. But better safe than sorry - just unplug it first.I'm really pleased with this little heater. It does exactly what I need it to do, it's inexpensive and works great. If you're torn between the 4 & 7.5 I'd monitor your water temp to see how much you need to increase it and choose accordingly.

I have had two of these 15w for both of my 3.5 gallon tanks for my betta fish for a year. I have about an inch of gravel that I buried the heaters under, they are centered in the bottom of the tanks. Each tank has a small hide and some random foliage. The tanks have a filter and a light. My home temperature varies between 64 degrees and 78 degrees throughout the year depending on the time of year and day. These heaters have consistently kept my tanks between 78 and 82 degrees. My bettas are very active, healthy and happy. I have not noticed any slime build up on them or the cord. Speaking of the cord, it’s long making it easy to reach an outlet and keep it buried in the tank. The suction cups it comes with help to keep the cord in position. I did try the smaller watt but it was not sufficient enough and didn’t heat the 3.5 gallons of water more than 72-74 in my home during the summer when my home was generally between 76-79 degrees. I recommend this 15w for any tank up to 3.5 gallons more than that and I can’t say as I haven’t any tanks more than the 3.5 gallons. The tanks I am using this in are the Top Fin: Enchant 3.5 gallon.

I've purchased 3 different types of Betta heaters for the various Betta I take care of. This particular heater advertized that it "Helps guarantee your fish a healthy life" but let me tell you how wrong they are. This is the only heater out of the 3 that I've used that has actually burned my Betta's fins... Quite badly. He was a beautiful fish and it was sad to see him suffer like that. This is in no way a safe heater for your Betta. I don't usually like to write 1 star reviews, but I really want to warn those of you out there looking for a heater not to get this particular product. It will damage your beautiful Betta if not kill him. The other heaters I'm currently using are the Marina 8w Betta heater and the National Geographic 10w & 25w heaters. They don't burn the fish in any way and they are fully exposed to the fish. I hope my fish can survive the burns... Please avoid this product if you want your fish to actually have a "guaranteed healthy life."Update (5 days later): I removed the heater immediately after I noticed my Betta was burned (about 3 days after I got the heater.) At first I took my Betta to the local exotic fish and reptile store so they could tell me what happened to the fish and how to help it. They said that the Betta appeared to have been burned by the heater and informed me that they've seen this before. Apparently this frequently happens when people purchase a cheap heater over the internet. The store employees then recommended a product called Melafix by a company called API. They told me it was a 7 day treatment and could help regrow his burnt fins while preventing infection. However, 5 days later my Betta passed away from its wounds. I'm really sad about it. I hope this story prevents this from happening to a future Betta owner.

I setup a 5.5 gal Betta tank on my desk at my office and since the tank was smaller I wanted to try to hide the heater as much as possible. I decided on the Hydor since it was fully submersible and was told the heater cold be buried under the substrate. The heater arrived and upon first inspection everything looked good. The heating pad was nice and wide and even pliable. The cable was long and came with a cable guide and suction cup to help route the wire. Once in place the heater appeared exactly as I wanted, invisible except for a cable running up the side of the tank.Once it was in the tank initially everything was fine and the the heater would get the tank to a respectable temperature such as 78 or 80. Over the course of a few weeks and season change I noticed the temperature would fluctuate from 68 to 84 degrees. I will not fault the heater for the fluctuation. The heater did exactly what it was advertised to do and did it well. The wide fluctuation is entirely do the the ambient temperature in our office. During the winter months the ambient temperature is very warm and this caused the tank to get hot since this heater is always on. Now that summer has come the AC is running and it s very cold and the 15w heater just doesn't have enough power to heat the tank.Pros:SlimCan be hidden.Under the right ambient temperatures I feel like this heater would work fine for small tanks.Cons:There aren't many cons to this product however it seems the useful application of this heater is very limited.