I've shipped many fish shipments. For USPS Priority, I'm in a habit of labeling them "Live Fish", but no longer mark my Express shipments.
For Express USPS shipments, I've stopped marking "Live Fish" on the box, and have asked the counter USPS clerk to stamp them perishable/fragile. Why do I no longer mark USPS Express boxes as "live fish"? It's because USPS Express can rely on commercial airlines to take the boxes from airport to airport in cargo. And some commercial airlines will not accept tropical fish or various other reptiles/fish/etc. So as I have read and understand, it's the commercial airlines' right to deny a USPS Express box in cargo if they have any reason to believe the shipment contains something on their prohibited list. Has nothing to do with USPS at that point, and I do not want to risk a box being rejected and delayed until it can get matched up with an airline that will take it.
I'll still mark "Live Fish" on the Priority boxes because I actually do have faith that USPS people are not heartless jerks, and that they will try to take care of a shipment. I do not expect for USPS priority packages to possibly rely on commercial airlines for transport like they might with Express service. I generally expect that USPS folks will handle the package start-to-finish and figure if it's marked and it catches their eye, they'll handle it more carefully.
Just my opinion. :-)