Severums

hi i was just wondering, if anybody can help, i have 2 large severums, been fine since i had them i did have them with parrot fish but sold them because i got told i couldnt have much with parrot fish, about a week ago the severum started picking on the other one nipping at its fins and bashing it around the tank, poor thing all i have in my tank is a large plec and a baby plec and the 2 severums, i have a 4foot bullet shaped tank 300litre if any1 cud help that be great thanks claire (the one that is bulling is the one without the black stripes on it )