Review on Hydor Slim Heater for Bettas: Optimized for Bowls and Aquariums by Josefina Rodriguez

Pros: 1) Inexpensive and easy to install 2) Works well in 3 gallon water CONS: 1) Always On: Won't turn on/off if it's a certain temperature reached, it will further increase the temperature in a small volume of water. ***THAT YOU MUST CONTROL THE TEMPERATURE CAREFULLY, ESPECIALLY IN A WARM ROOM AND/OR IN THE SUMMER!*** 2) It is NOT RECOMMENDED to use this heater for less than 3 gallons, see notes. 3) Evenly with gravel up and lots of slack in the power cord, this heater tends to "float" to about the gravel line no matter how deep I put it under the gravel (should be about 3/4 inch gravel at the top). CONCLUSION: Solid BUY if for 3-5 gallons of water. If you are looking for a betta tank heater for this amount of water this is the one for you. NOTES: Purchased to initially heat a 1.5 gallon Tetra tank (see review) Tetra 29137 WaterWonders Aquarium Cube, 1.5 Gallon, Black and it raises the temperature too high for the Betta (up to 87F in a room of about 75F). Moved the fish and this heater to a 3 gallon, 3 gallon tetra cube aquarium kit and the temp fluctuated between 77-81F throughout the day. I'm not sure if it's too "wild" temperature swings over 24 hours, but my betta doesn't look worse from wear and tear or stress. I will update this review if anything changes.