I keep a Firemouth pair sub-adults with my Oscars. Thing is I barely fed my Oscars with live fish (out of concern of disease risk and low nutritional load). As with many issues around Oscars, what works for a particular fish (or pair) might not work for other(s).
Unlike Kenyi, the only small sized fish that dares to confront my Oscars, the Firemouth behavior is more in the line of the Convicts, showing no fear but having no problem to retreat and if possible avoid the bigger fish.
Managing aggression can be tricky. I have a heavily planted tank, with plenty of cover -driftwoods, clay pots, river rocks. Somewhat careful male to female ratio (when possible) and keeping a plan B (another tank) at hand.
My Firemouth pair was introduced into the tank after a considerable water change and partial tank decor re-arrangement (I left some areas untouched as they were already conveniently "housing" other fish).
Then again something can work for months and then not anymore. Heros Severum are tried and true tankmates for Oscars. I had to remove my pair due to nasty harassment and some not-so mild bruises my Oscars inflicted in them. No problem since I had my plan B at hand.
Pepe
Santo Domingo
