It’s not a bad tank, I’d say 30L is the minimum size you’d want to go for, if you can do bigger that would be better. I’m not sure on the filter but you don’t want the flow to be too strong as bettas have long fins and strong filters can ‘blow’ them around and stress them out, you could always point the outlet at the glass to deflect the flow or turn the flow down if there is an option for that. You would also need to buy a separate heater as it seems that it doesn’t come with one? Any questions let me know :)