Right now I have a 7 inch bullhead, 7 inch JD, 1.5 inch Cory, 2 inch bluegill, 5 inch sunfish, 4 - 2 inches clown loaches, and 2 - 1.5 inch convicts in my 150. Most of the fish including the bullhead have been in there for about a year.
A bullhead of that size cant fit a 2in JD in its mouth. My 7 in bullhead would have a hard time fitting that in his mouth.
Both fish have had about the same growth rates but the JD has recently slowed quite a bit and the bullhead has kept going so that shouldn't be a problem.
The jd and cons both seem to be very aware of what a catfish is and many times chase him around the tank.
I previously had a 10-11 in bullhead in the tank with most of these fish. Only once did he harm any of the fish and that was when I first got my convicts at about an inch long and one swam right in his mouth. So I can't fault him for that. I ended up having to remove him because the JD and a larger male convict both at about 4-5 in repeatedly beat him up.
This is why I think there is not an issue with keeping a 2 inch JD with a 5 inch bullhead.
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