Sooner or later they will fight to death. One day you might wake up and only one fish will be alive. I don't know what kind of filter you have. But if a strong current is present (plus the bubbler ) the fish could be a little uneasy and that is distracting them from each other. That doesn't mean it is ok to still keep them together. Bettas have personality that can accept other fish or not. But never to keep two or more Male bettas together. However, The best thing is to keep them by themselves, it depends on the betta itself.