I have 4 platies, 3 males and 1 female. My roommate didn't want them anymore so I offered to put them in my 55 gallon tank at home. I brought them home in April I think but my roommate had them over a year (they were born in her tank). Two of the males are VERY thin (the other male and the female are plump and healthy), like I keep expecting them to drop dead of starvation. At first I thought they were being bullied. The third male is the only one who has access to the female, if the others get close he chases them off. At feeding time if they eat near him he'll chase them away, the female will as well. But since they started getting thin I've closely monitored their eating and they get plenty, it's a big tank. They eat and keep it all down but are staying super, super thin. I figure parasites could cause this, but anything else I should be worried about?