Fuelleborni (I've seen 10"+ males) are by far the larger of the Labeotropheus (vs. trewavasae, which should only max at about 4-5")...and also more aggressive.
As a keeper of fuelleborni, I have to ask... are these 10" fish overfed captive crazies, or is that within the range of relatively natural possibility?
My understanding is that fuelleborni should cap out at 7" or so for males and somewhat smaller for females. My 2 year old marmelade cat males are in the 4.5-5" range and I would imagine will now grow very slowly towards the 6-7" size that I would expect.
What are we really talking about here in terms of natural vs. max size?
I will say that my fuelleborni live in a pretty rambunctious tank, and certainly have the capacity to be mean, but have a good enough ratio of females and well enough established hierarchy between the two males that actual violence is rare. The dominant male turns pale and shows at the subdom, at which point whatever threatened or imagined menace goes away instantly.
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My advice is based on the fact that I'm a strong believer that environment, tank size, and planning can address a lot of aggression issues and that with good planning, you can and should keep the fish you really want. I keep some relatively unpleasant fish so would say, be ready to bite your nails, be ready to buy some more tankmates, be ready to sell some males and buy females at a relatively higher cost... but if you love a fish, find a way to keep that fish.