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#1
This is typical behavior for most Geophagus species, they live in social groups in nature and will have a hierarchy within the group. So when you had 4 of them they were comfortable with their group size and had a group hierarchy in place.
Now with only a pair, you probably have a dominant male, which is larger and showing brighter orange/red coloration on the head, and a sub dominant showing little to no orange/red coloration and smaller in size. So basically just the big guy pushing the little guy around all day and no other Geo's to spread his interest, aggression, etc. between.
Best bet would be to pick up a couple of more Geo's and get them back into a more comfortable social group size and the aggression issues will dissipate.