In my divided betta tanks I have an Elite filter in each section. They're small, submersible and silent. The water flow can also be adjusted on them. Makes me feel better knowing each betta has optimal water conditions. Even though the dividers have little holes in them it still doesn't let the water flow through it properly so whatever section the HOB is on that section will get the most filtration and the other sections not so much. I've just found the Elite filter to work the best for divided betta tanks. I would think the tiny whisper in-tank filter would also work very well if you had one in each section.