I bought this pump for the Fluval Flex 9 gallon tank. The fluval pump was loud and rattling no matter how many times I tried to fix the impeller. This was recommended to me for this tank, and it works great! It is SO much quieter. I have this tank in my bedroom and it's no longer bothering me. It's also adjustable, which my betta appreciates! Little note, it does fit into the back chamber where the pump is supposed to go, but it is a tight fit. I have the long fluval heater and that would no longer fit with the pump. Had to move it to the other chamber. Have had no issues with maintaining the temp this way.
Excellent little pump and is far more powerful than one might suspect compared to its extremely small size. German engineered and quality made. I've never been disappointed by an Eheim product since I bought my first Eheim filter in 1984 that I'm still using 38 years later.
Piece of junk, and only works with very specific sized tubing. Thank goodness for Chewy and their great customer service.