When can you reliably tell the sex of a swordtail?

[lewk]

A few months back, I bought a group of Cauliflower Swordtails from an importer. He estimated that they were right around the 3 month mark at the time. Based on what I had seen from Greg Sage on Select Aquatics videos, I decided to avoid the small number of fish that had already sexed out with swords at that time, as ideally I wanted a big bodied male as my breeder rather than an early maturing one and I am fine starting out with a group of few males and many females. Now, about 3 months later, I still haven't had any clear signs of swords or gonopodiums. I'm not sure if I should be patient and wait longer for my swordtails to sex out, or if I should start the search for a male now. 

[gardenman]

It can take a while. Some males are absurdly slow in developing. I put one male and two fully grown presumably female neon swordtails into my quarantine tank to keep it cycled and give me a backup colony. After about a month one of the fully grown females became a male. Any male you buy from a retailer is likely to be an early developer as they don't keep fish around for a year or so. And it can take a year or so for a slow developing male to emerge.

Having bought just three fish, you put yourself in an iffy position. Five would increase your odds of getting at least one pair. The more young fish you buy the better your odds are of getting a pair. Buy a thousand and the odds of them all being the same sex is ridiculously small. 

With fish where there's a size difference between males and females you really need the right person netting your fish. Some good sellers want to give you the biggest fish they have available, but by doing so they may inadvertently give you all fish of one sex. A later buyer who comes along after all of the "big" ones were sold might get all of the opposite sex. If you're planning to breed fish and have the option to talk to the catcher, try to get a mix of sizes. Ideally, you'd want half bigger and half smaller. That improves your odds. And the more fish you buy, the better your odds get.

[lewk]

On 2/11/2023 at 6:48 AM, gardenman said:

It can take a while. Some males are absurdly slow in developing. I put one male and two fully grown presumably female neon swordtails into my quarantine tank to keep it cycled and give me a backup colony. After about a month one of the fully grown females became a male. Any male you buy from a retailer is likely to be an early developer as they don't keep fish around for a year or so. And it can take a year or so for a slow developing male to emerge.

Having bought just three fish, you put yourself in an iffy position. Five would increase your odds of getting at least one pair. The more young fish you buy the better your odds are of getting a pair. Buy a thousand and the odds of them all being the same sex is ridiculously small. 

With fish where there's a size difference between males and females you really need the right person netting your fish. Some good sellers want to give you the biggest fish they have available, but by doing so they may inadvertently give you all fish of one sex. A later buyer who comes along after all of the "big" ones were sold might get all of the opposite sex. If you're planning to breed fish and have the option to talk to the catcher, try to get a mix of sizes. Ideally, you'd want half bigger and half smaller. That improves your odds. And the more fish you buy, the better your odds get.

Thanks for the reply. I bought seven fish, figuring at that age I would still be giving myself a pretty good shot at a male. I've lost one since but the remaining group is looking really healthy.

As far as selection goes, for better or worse I chose them myself. The whole group was fairly uniform on size, though, and the traits I put the most focus on were conformation, color, and dorsal. Hopefully I didn't screw it up.