oscars are aggressive fish, so you wanna watch out for that too [if you get an oscar too large]. also, oscars grow very quickly. if you wanna grow this guy out in a seperate tank for a month or two, he should be fine. it's not necessarily if the oscar is smaller than the bichir that he will be lunch, because bichirs are longer than they are large. but this guy seems too small. anyway, to answer the fin question, as others have already said: yes, they will grow back. as long as it isn't into the base of the fin. this happened to my smaller oscar [puffer nipped his tail about half a second after i put him in the tank] and it grew back very very quickly. i put in some aquarium salt to a little less than the recommended dose and it seemed to help a bit.