A well cared for and appropriately housed Oscar should reach well over 8", if it doesn't reach 10"+ then something is wrong.
I definitely feel that a well fed Oscar would not eat a full grown senegal especially if the senegal was bought at near adult size and the oscar small. Of course there will be individual fish that would give it a go but in general terms more often than not it should work. Make sure there are hiding places such as pipes for the senegal to retreat to though.
I definitely feel that a well fed Oscar would not eat a full grown senegal especially if the senegal was bought at near adult size and the oscar small. Of course there will be individual fish that would give it a go but in general terms more often than not it should work. Make sure there are hiding places such as pipes for the senegal to retreat to though.