Hello and welcome to the forum!
Do you currently have your betta and cherry barbs living in a 1 or 2 gallon tank together? Usually bettas shouldn't have tankmates in anything smaller than a 5 gallon tank, and even at a 5 you'd be looking more towards snails or shrimp. If the fish are overcrowded, it could cause aggression and might explain the fin biting that's happening. I'd suggest picking out a larger tank for the cherry barbs for sure, at the very least a 10 gallon, depending on how many you have.
You mention a filter that doesn't work well. Is this because its too strong? You could look up DIY filter baffling (a lot of people use a plastic bottle and cut it and hang it over the filter to displace the water outflow if its too strong), or you could use a sponge baffle! If the filter itself is okay and its simply too strong, those would be easy fixes!
I'll try getting a bigger tank for the barbs. Can you send me an image of how you can put a strong filter in a plastic bottle? Thanks for your help!!!