its a very good concern and the answer is Yes, the weak one can cause you problems. Fish like these act as incubators for disease organisms and can shed them into the water in numbers that can eventually overwhelm healthy fish immune systems. If it were a fish that was a healthy runt, its one thing and a matter of preference on the culling, but if its weak and sickly,You should cull it or hospitalize it and try to heal it.. but culling is your safest bet.My concern is that it's overall poor condition could leave the system open to assorted illnesses since, as I have learned over the years, disease is always present in a mature system and it is only the fishes health that prevents outbreaks and this one is not healthy. Nor does it look as if it will thrive so...should I kill/cull it?
al