Tanks A and B (Treatment 1). Stage 1: three fish individually kept in three serially adjoining compartments per tank, separated by plastic mesh partitions. Stage 2: certain resident males were exchanged for stranger males. Tanks C and D (Treatment 2): communal tanks for 3 fish each, after 3 weeks plastic mesh partitions separated the fish. Individuals coded RM, "resident" males; SM, "stranger" males; M, males; F, females. F and M were kept in groups even before Treatment 2 started.