I use filters in all 3 of my tanks, 7.5G, 13G and 37G....all fluval filters.
For my 7.5G tank which is divided and has 2 Bettas in I use the baby one of the Fluval filters...it`s called Fluval Plus 1.
It doesn`t have a flow adjuster but to be honest I don`t find that the flow is too much at all for my Bettas to handle, they cope with it really well.
I have planted to the front and side of the filter and find that interrupts and lessens the flow too.
Here`s a link to the smallest one i have:
Fluval Plus, Fluval Internal Filter, Fluval 1, Fluval 2, Fluval 3, Fluval 4
Hope it helps.