Boy or Girl?

I had a customer come into the store today, intent on getting boys and girls off all the fish that he wanted.
The problem?
I can't always tell the difference between boys and girls in the fish.
Basically almost all the fish.
Because all of them are young...and aren't yet full grown.
And also because most of the fish are egg layers and they're just...difficult to tell apart when they're babies.

And this customer wanted to know if I could tell him the difference with the catfish.
Which I could not.
And I told him so. That the catfish were too young, that I had no idea. (honestly I've never looked it up so I don't have the first clue as to what to look for)
But for whatever reason, the customer didn't seem to comprehend that when I said I had no idea for all the catfish.
That I literally meant ALL the catfish.
As he pointed to one tank of Albinos and asked if I could tell the difference and I said "No, I can't tell for any of the catfish." 
and he proceeded to point to every single tank of catfish after that point. (five tanks in total) and ask me the same question. 
*exhales* 
No. No. No. No. What do you not understand about me saying NO? 

*shakes head*
He ended up getting a couple anyways.
But do I know what gender they were? 
Not the foggiest. *shrugs* 

-Sarnic Dirchi