Betta aggressive with Cory?

Question
Hey
I have a question about a Male Betta I've had him for a year now,and he is in a 30 litre bi orb which I removed the filter from and just filled the bottom with gravel, I recently had to place him in an 80 litre tank with a cory and and a cardinal tetra as I used his tank temporarily as a quarantine, the disease affecting the quarantined fish only affects that species so I managed to get a smaller tank set up which they are in, did a total clean of the 30 litre tank and moved the betta back in there, he's fine. But I've had to move the Cory and the cardinal tetra into the Betta's tank while I clean the larger tank. The problem is he's okay swimming around but if he gets close to the cory he attacks him and the same about the cardinal tetra. He's currently in a bag used to transport fish, but still in the tank. And I'm unsure what to do, in the bigger tank he was incredibly passive staying away from the others but now he's back in his own little tank he's being aggressive I know it's their nature but is there any way to stop this?
Thank-you

Answer
Liz,
Unfortunately no there is no way to change this. In the larger tank, he had plenty of room to "guard" his territory. When placed into the smaller tank, he is now competing with other fish in smaller swimming space. I normally tell people to keep the male Betta's by themselves. That way, this type of problem does not arise.