ive got a similar size tank with 3upland bullies, took the little koura out so fish can feel more secure and that tank alone is probably too small for koura, especially when bigger. added 3 shrimp instead ,so far all good
just need to keep an eye on temps, it can go up quick with small tanks, and keep water clean as possible. im wondering if my bullies may outgrow that tank actually. may just end up a shrimp tank.