Actually you are wrong, there is a risk that a bullhead may get eaten or killed by a piranha even if its a solo RBP as the new bullhead (especially adult bullheads) may not use the snug cave on first night. Bullheads do come out in the daylight in both wild and capitive. Personally I won't house anything with piranhas beside very small fish or heavily armoured catfish.^actually I wouldn't think so. He the op building a cave for the catfish (a snug cave) I think he could pull it off since the bull head would mostly come out at night to feed in the beginning. The rbp will try but eventually give up when he comes out in the day to
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To OP, do not get a bullhead as a tankmate for your piranha.