Red-tailed sharks can be quite territorial. One thing that occasionally works with territorial fish is to remove it for a little while, rearrange the tank slightly, and reintroduce the fish. This sometimes fools the fish into thinking it's a new environment, where it doesn't feel as defensive over its space yet. But to be honest, I don't think that will help too much with those fish in that size of tank.
While we're on the topic, bala sharks get entirely too large for a 30 gallon tank

They can reach up to 16" given the right amount of space, and they do best in groups. Is there any way you can either provide the bala with a larger home and some tank mates or take it back to the store?