Keyhole Cichlid & Pearl Gourami

I have had my tank for 8 months now. It's a 120L Juwel Aquarium. In then last
two weeks one of my pearl gourami's and my keyhole cichlid have been butting
each other and being aggressive towards each other. when this happens, my
keyhole cichlid changes to a darker colour across large areas of his body.

I have had both for several months and this has only just started to happen.
The only thing that has changed recently is that 8 fish have been removed from
the tank.

Is this normal for the fish in general, or is it due to the extra space. And
should I separate them

Any help would be much appreciated as up until recently they have been fine
with each other.