Culling Aquarium Fish

How to cull your freshwater aquarium

Killifish Fry
Christopher Green

If you’ve ever bred fish, either intentionally or accidentally, you’ve probably been faced with the problem of having way more young fish than you can handle. Perhaps you’ve been told to cull the juveniles early on. What does that mean, and how do you go about doing it?

What is Culling?

Simply put, culling fish is the process of weeding out some of the young fish to reduce their numbers to a level that is appropriate for the size of the aquarium.

In some species of fish, it is not unusual to hatch out literally hundreds of fish. Few aquarists have the space to house that many fish, which means something has to give. Either the fish must be culled, or the entire clutch may be lost due to deteriorating habitat conditions that result from overcrowding.

Isn’t it Cruel?

Anytime a living creature is put down, the question of cruelty invariably arises. Yes, it does seem cruel, but in nature, the same thing occurs naturally. Fish are one of many species that produce excessively large numbers of young, many of which will never make it to adulthood. If all the young were to survive to reach adult size, the available resources would be depleted, upsetting the ecological balance. The young fry that ​don’t make it are a source of high-quality protein to other animals. It is the circle of life, and far from being cruel, it is quite natural.

How to Cull

Culling aquarium fish may seem gut wrenching, but if you approach the task with the natural process in mind, it isn’t all that hard.

In nature, the smallest and weakest fry would either die or be eaten by predators. Likewise, deformed fry would usually meet an early demise at the mouth of a larger fish. Those are the ones you should cull out.  

Another determining factor may be the ​coloration and markings of the fry. Although they don’t always display full coloration or markings at an early age, you will usually be able to pick out some that are drabber compared to the others.

Those are candidates for the cull.

After identifying the malformed, paler colored, weak, and smallest fry, separate them from the others. You’ll find that often they will be easier to capture than their stronger counter parts. Once separated, dump them in the tank with adult fish, which will be delighted to have a live meal, and will benefit from the nutrition. The remaining fry will have a better environment, and as a result will be more likely to grow into healthy adults.

Remember that when culling does not take place, fry tanks often suffer heavy losses, due to overcrowding. By culling, the best fry are given a better chance. It’s the right thing to do – not a cruel practice.

Options to Culling

If the process of culling still seems far too barbaric for you, there are a few options. The first one is to set up another aquarium (or two, or three) to house the fry. Separate them early, to give them as much room to grow, as possible. Keep in mind that unless you plan to get rid of these fish, you are likely to have more fish on your hands once they grow to adults and start reproducing.

Another option is to sell the fish, or simply give them away. Local pet shops will often take fish off your hands, as long as they are a desirable species.

Sometimes businesses that keep an aquarium in the lobby will also be willing to take excess fish. Last. But not least, check your local schools. Often schools will enjoy having some free fish for their classrooms.

As a word of caution, the one option you should never use to get rid of undesirable fish is to dump them in local waterways. Odds are they will be eaten by the local inhabitants, so you haven’t done them any favor. However, in the case of aggressive aquarium fish, they may wreak havoc on the native fish. Be mindful of preserving the local habitat, and don’t dump unwanted fish.