In that 55 with all those fish, I'd be doing 30% water changes every day, no wonder the arowana was attacked.
Another thing, arowanas are quite sensitive to nitrate buildup, so even if the oscar didn't kill it, under those over crowded conditions, the stress, and eventual decline in water quality probably would have. Overfiltration does not help reduce nitrate, only water changes will, and/or a sump filled with plants sometimes makes a dent.
You are lucky the fish are small, although a 6" oscar is not small, for that tiny tank.
Another thing, arowanas are quite sensitive to nitrate buildup, so even if the oscar didn't kill it, under those over crowded conditions, the stress, and eventual decline in water quality probably would have. Overfiltration does not help reduce nitrate, only water changes will, and/or a sump filled with plants sometimes makes a dent.
You are lucky the fish are small, although a 6" oscar is not small, for that tiny tank.
