The betta-in-a-vase is a very bad environment for a betta for many reasons.
But let's focus on your current fish...
The filters on the miniBow1 are very strong. Is your fish able to swim without being tossed around the tank? I had one of these and removed the filter because my fish couldn't swim straight when fighting the current and he had to race after his food.
I'm not saying to completely discard the filter because I know they are a convinience and reduces water changes but you could slower the flow a bit by placing a piece of pantyhose around the intake of the filter or by placing a piece of bubblewrap underneat the outtake of it.
Do you have the mini bow with the filter under the gravel or the one with the filter over the water?
And as other members said, you do need to regulate your temperature. Too cold water will keep your fish lethargic.