jediknight wrote:I keep them in a 2100 galon tank, that tank I used to keep big stingrays... now with these 3 oscars... hope have at least 1 male and 1femal
regards

is it 2100 gal tank??? can you describe the dimension please?
venting is one way but it will be very stressful to the fishes. moreover4,
it is not 100% fool proof... thus it is not practical to do at all.
if by any chance, you are moving the fishes, you can hold them for a while
and look for the markings near the anus...there is a circle the looks like
an oo for a male and an Oo for a female...but as a i have said, this is not
fool proof and one must be experienced enough to see the size difference.