The honey gourami are very peaceful, but males are territorial just like their cousins, the dwarf gourami. They will establish a pecking order in the tank. Given that the honey gourami spends most of the time in the upper levels, and C. ansorgii on the lower levels, I guess the honey gourami just found the plakat to be a nuisance to his dominance of the upper layer.
I would watch things closely if I were you. On most days, plakat will probably murder the honey gourami.
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