This is a very tough part of breeding bettas, and a very good reason not to breed them in the first place.
With each average betta spawn being 500 young fish, even modest culling means you’ll be killing 250 little betta youngsters. Even keeping 250 is difficult, because you’re going to have to house, feed and clean 250 bettas. If you know of the community event that needs party favors or tokens, you might be able to unload a couple dozen of your bettas there.
If possible, arrange for at least 50 new betta homes before you breed your fish unless
a) you don’t mind killing off 90% of a spawn
b) you have a very, very large fish room
c) you have a standing arrangement to sell the fish to a retail vendor
Assuming the spawn (the group of baby bettas resulting from your breeding effort, or, more precisely, your fishes’ breeding effort) is about 500 fish, and assuming you can’t possible keep more than 200 of those fish, you’re going to have to kill, or “cull” 300 fish.
To do this, you’ll first need to separate out which ones you want to keep, and which ones you don’t. For the purposes of this example, I’m going to assume all your spawn were raised together, so they can continue to be kept in the same tank.
Get two large containers. One will be for the fish you want to keep, the other for the fish you don’t want to keep. If you’re a softie like me, you may want a third tank for the fish you aren’t sure about.
Get a good light, and if necessary a magnifier glass. Get a container where you can very, very closely see each fish. You’re going to give each of those 500 fish a close once-over to weed out the losers.
The first cull can be to get rid of the sick and clearly malformed fish. These should be the ones that are clearly not nature’s favorites, and hopefully putting these guys in the cull tank won’t hurt to much. Remember – in the wild, barely in one five would survive anyway. You’ll probably lose 10-15% of your spawn in this cull.
The next cull can be to remove fish with undesirable colors and even modestly malformed fins or other body parts.
The third cull can be to weed out the remaining fish that are nice but that you just don’t have room for. This will be the hardest cut. This is when it might be good to think of local pet stores, local teachers… anyone who might be able to find homes for even some of these perfectly good fish that just quite up to your breeding standards.
To actually kill the culled fish, get a bucket and fill it half with water and half with ice cubes. The water should be extremely cold… almost cold enough to freeze on the top. Then net the to-be-culled fish and drop them, batch by batch, into the icy water. This is the easiest and most humane way to kill them. The fish will go into shock upon contact with the water, and they will be knocked out in a second or two. They won’t feel the rest. At least that’s what the medical information says.
