I agree with Teelie on all counts there.
RTBS's are quite quarrelsome as they grow.
TB's really need to be kept in large groups in order to keep their inate aggression amongst themselves, at least 6 ideally more fish. Your tank is not big enough for a group that size. Also, TB's are very active swimmers, and a small tank like that is never going to be really suitable, even if it were a species tank.
If you do, indeed, have a Common Plec, they are well known tank busters!
A sexed pair of Cherry Barbs, or possibly a trio, 1 male 2 females would work. (2 males may fight, especially in small tanks.)