Thankyou for your reply, I had wondered about injury as I had put the convict in with them as he had been bullying the smaller ones. Being a quarter of the size I thought the Acaras would put him in his place. He doesn't seem to have any fear of them at all and after a couple of weeks is cruising the tank, so maybe he caused an injury overnight. I'll remove him as soon as I get home from work. On closer inspection the fin did look a little damaged so I have put in some Triple Sulph to help the healing and prevent any bacterial infection. I'm a little wary of hospital tanks as the first and only time I used one, I nursed the fish back to health, all fins back nicely, just to have it aniliated as soon as it was put back in the main tank of Africans!!!!
Thankyou again for your advice