Hydor Betta Bowl Slim Fish Tank Heater, 7.5-watt, measures 5.4L x 2.5W inches oval, 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.

It heats a Tetra 3 Gallon Cube aquarium between 3 and 5F. It has no OFF (if it's plugged in it's on), so I would remove it in summer if you're like me and keep it at 77 in the house while you're at work. It doesn't heat well if it's got more than a single layer of gravel or sand overtop. If you have more than 3 gallons, I would suggest a bigger heater. I keep an Aqueon MiniHeater 10W in the 5 gallon that almost works too well. It, too, has no auto off, but it heats the 5 gallon between 5 and 7F. This one has the benefit of being on the bottom of the tank, which makes for more even heating for the poor circulation of the tiny filter that came with the Cube. However, I have an undergravel filter in the 5.5 gallon (glass, Aqueon) which seems to do a good job of circulating the heat.

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 have this in my Beta bowl. It's not a huge environment, but with a fairly large surface area for it's size (i.e. it's rather shallow). Full, the bowl probably holds about 1 1/3 gallons, it's probably got about a gallon in it as I write this. I keep the heater buried in the gravel (well, really in, the gravel is stacked around, but not over it) and the cord runs up the side of the bowl attached with a suction cup (which came with the unit). I don't keep daily temperature logs but a quick look at the thermometer in the bowl indicates it's at about 74F right now. I would guess when full it keeps it at about 70F. The thermometer probe is attached opposite the heater. The whole unit is in a heated office, kept at room temperature, with air circulated by overhead fans, so it's a pretty stable environment. Since the heater is running flat out all the time, and has no thermostat, the temp would obviously fluctuate with the surrounding environment. Overall, the heater seems to work well for what it is. It's unobtrusive enough to not bother my fish, even in the small bowl, puts out just enough heat to keep him toasty, and I like that I can just jerk it out and set it aside without burning it out when I need to clean the bowl (it is not supposed to be damaged when not submerged and the cord slips out of the little suction cup clip, so you don't even need to undo the suction cup). I'm happy, and my fish is happy, but admittedly my needs were not great. In a real tank (or even a larger bowl), or in an environment where the temperature fluctuated, it would obviously not work as well, if at all.