Over the years, I had read up about this and also met quite a few Oscar breeders. The short answer is that it’s virtually impossible to tell, without using an endoscope.
The long answer is, that there are many theories.
Males :
– have longer dorsal fins that over lap with the tail.
– have three dark spots at the base of their dorsal fin.
– have more red colouration.
– have longer vent that does not protrude as much.
– have longer faces/head.
– exhibit greater aggression.
All this really relies on you having several fish, with one of each gender, for comparison.
Some swear by these methodical and some say they’re fallacy.
I think sometimes even the oscars get it wrong because it’s quite common for females to pair up with other females! Unless it’s because there’s a high incidence of homosexuality in this species.
For me, I won’t actually know until I see them lay eggs.
