We have four fossies, two are mature males that both colored up nicely and are 12" one mebbe a bit larger, the other two we are growing out hoping for females and they are just under 6" and show no signs of being male.
One of our males is mean. To the point that he will pick on any other hap in the tank except for the c. moorii. He goes after our 10" male venustus relentlessly as much as the other male fossie. I guess there is one with an attitude in every crowd. He does not bother the mbuna or smaller haps, not sure why.
When they really get it on the lids get knocked off and scaping gets rearranged.