Its quite strong on such a small tank. I have it flowing left to right. I'd think its probably too much for a betta as they like minimal current don't they? This is actually rated for tanks up to 60 litres. This tank is just shy of 5 litres There is a smaller version (Mignon 60, for 15 litre tanks).
It can be reduced slightly. There is a knob on top you can turn to reduce the flow but it doesn't make a great deal of difference. I originally stuffed some filter wool on the cascade which reduced it quite well. I don't think I'll need to bother as I'm not sure if they'll be any livestock in this, maybe some cherry shrimp (if I ever manage to get hold of some!) For now I'm just happy making a mini scape and I'm hoping I can keep it going successfully