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

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 read so many reviews on small heaters my head started to spin so I finally decided to get two of these for my betta bowls. I have had them in for about 24 hours and they have made my two bettas extremely happy.I have two vase-type bowls,yep I know they "prefer" more space but this is about heaters, not betta habitats. They hold approximately 1 gallon each. I have river pebbles and bamboo in each as well. I thoroughly cleaned the bowls, rocks, and heaters. Then placed one level of pebble, the heater, and more pebbles on top. I refilled the bowls, added some aquarium salt, and put the bettas (Todd Gurley and Dash) back in their bowls. I plugged the heaters in and watched, with trepidation, for any signs of overheating or distress.Well my two boys are happier than I have ever seen them. The water is about 78 degrees (the house is 70), so it is spot on as advertised despite the bowls being smaller than recommended on the packaging. My fish are swimming around, blowing bubbles, and eating like fiends. I've had to increase their feeding schedule due to their increased activity. TG likes to spend time right over the base of the heater while Dash prefers to sit on his curled bamboo ledge.The heaters fit nicely and are not very noticeable. I have the cords behind the bamboo and suctioned to the edge. I am very pleased with these and, honestly, if they only lasted 6 months-year I would still be happy and purchase a new one. I don't foresee that being a problem as I will remove them once it gets warmer and leave them out during the summer.

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.

Worked great until it died 3 months after purchase. Going to find another heater.

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.