Hi panda_cory
Actually, while there are some exceptions, many corys are easy to tell apart. Let me use C. pandas for an example:
This is a pair of mature corys from one of my breeding groups. The female is the larger fish on the left and the male is the small one on the right.
When I'm buying fish that are very young, or a species that I'm not familiar with, I have a strategy that seems to work out fairly well for me. First, I often buy corys at my lfs which is owned and run by the same fellow for over 30 years. When he waits on me, I tell him how many of each gender I want and let him use his judgment. I never go wrong when I do this.
Otherwise, I pick them out myself and ask the clerk to put them into a specimen container so I can see them as I go along. I can also look down on them. First, I will pick out my females. To do this, I ask the clerk to pick out the biggest corys in the tank. Unless they have been in the store long enough to be picked over, these will be females. Then, after having looked them over well, and trying to remember their shapes, I pick out the males from the ones that are left. They will be slimmer and often shorter. When they have been put into the specimen container with the females, I look at them again to be sure there is a difference. If I'm happy with them, I have them bagged up. If not, I ask for one or more of them to be switched.
I was thinking about getting a big group of them and wanted to get some female and some male.
They are incredibly difficult to sex and it is impossible when they are young - which they will be when you buy them.
