One constant problem with oscars in small tanks, is the ammonia content of the water. I've heard of people overstocking cichlid tanks to reduce aggression, but actually only mbuna tanks, and not new world tanks. Even if someone has been keeping alot of oscars in a relatively small tank with no problems doesnt mean that problems wont arise in the near future. And if he has had those 5 inch oscars for 5 years then they are obviously stunted, they should be putting on about 2 inches a year in a healthy environment. I am sorry to say it, because I think people underestimate stocking capabilities, but your friend is most definitely overstocked. I mean, it's better to give away or sell half your fish than it is to lose them all to disease or suffication.