I got this one
and love it! The flow is totally adjustable, it's SILENT (Alpha's tank is in my room so that was a must), and it's really cheap! It also comes with a prefilter sponge on the intake which makes it great for bettas and also for my little shrimpies. And it's really well, slim! I have 2inches of clearance behind my tank and the filter fits there. It's about the size of an average toothpaste box for width & length; height-wise, I'd say it's about 6inches? idk, I'm not good at guesstimating size lol. It comes with a goldfish bag & a tropical fish (carbon) bag that can be replaced with the Tetra Whisper small biobags, and/or you can put filter sponge in there to colonize your BB. Oh! And the intake is adjustable inside t he tank so you can raise or lower it according to your tank size. And if you have/sign up for the 1 month trial of Amazon Prime, you get free shipping!
As you can probably tell, I reeeeeeeeeally like this filter. I went through about 4 filters before I got this one which I kept.